Issues with LAN connections?

Other player disconnects randomly every minute or so. He gets a large amount of lag, the kind where nothing happens when you eat or place blocks and such because of server disruption or something, and he gets disconnected. When closing the LAN world, my client crashed as well. It simply stopped responding, so I didn't get a crash report, but I do have the log file from that particular session.
When we tried earlier today, he was disconnected several times, and upon closing the world, it did not crash. When we tried a few minutes ago, he still disconnected but we did not try again. My client crashed as well the second time only.

The world we were using is a 1.8 customized world. The preset is:

{"coordinateScale":684.412,"heightScale":1.0,"lowerLimitScale":512.0,"upperLimitScale":512.0,"depthNoiseScaleX":200.0,"depthNoiseScaleZ":200.0,"depthNoiseScaleExponent":0.5,"mainNoiseScaleX":5000.0,"mainNoiseScaleY":5000.0,"mainNoiseScaleZ":5000.0,"baseSize":8.5,"stretchY":12.0,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":5.0140843,"biomeScaleOffset":0.0,"seaLevel":63,"useCaves":true,"useDungeons":true,"dungeonChance":100,"useStrongholds":true,"useVillages":true,"useMineShafts":true,"useTemples":true,"useRavines":false,"useWaterLakes":true,"waterLakeChance":4,"useLavaLakes":true,"lavaLakeChance":80,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":5,"riverSize":5,"dirtSize":33,"dirtCount":10,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":8,"gravelMinHeight":0,"gravelMaxHeight":256,"graniteSize":33,"graniteCount":10,"graniteMinHeight":0,"graniteMaxHeight":80,"dioriteSize":33,"dioriteCount":10,"dioriteMinHeight":0,"dioriteMaxHeight":80,"andesiteSize":33,"andesiteCount":10,"andesiteMinHeight":0,"andesiteMaxHeight":80,"coalSize":17,"coalCount":20,"coalMinHeight":0,"coalMaxHeight":128,"ironSize":9,"ironCount":20,"ironMinHeight":0,"ironMaxHeight":64,"goldSize":9,"goldCount":2,"goldMinHeight":0,"goldMaxHeight":32,"redstoneSize":8,"redstoneCount":8,"redstoneMinHeight":0,"redstoneMaxHeight":16,"diamondSize":8,"diamondCount":1,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":7,"lapisCount":1,"lapisCenterHeight":16,"lapisSpread":16}

The log file starting with the opening of the world was as follows:
[17:57:13] [Server thread/INFO]: Preparing start region for level 0
[17:57:14] [Server thread/INFO]: Changing view distance to 13, from 10
[17:57:14] [Server thread/INFO]: XAcidMystX[local:E:b57f1230] logged in with entity id 62892160 at (-295.0596412467403, 63.0, -901.0009518292493)
[17:57:14] [Server thread/INFO]: XAcidMystX joined the game
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 126430, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 243364, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 243364, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 243364, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 243364, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 243364, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 243364, defaulting to 0 (Ocean)
[17:57:16] [Chunk Batcher 0/WARN]: Biome ID is out of bounds: 243364, defaulting to 0 (Ocean)
[17:57:23] [Server thread/INFO]: Saving and pausing game...
[17:57:23] [Server thread/INFO]: Saving chunks for level 'Summer Of Death'/Overworld
[17:57:24] [Server thread/INFO]: Saving chunks for level 'Summer Of Death'/Nether
[17:57:24] [Server thread/INFO]: Saving chunks for level 'Summer Of Death'/The End
[17:57:27] [Client thread/INFO]: Started on 52789
[17:57:27] [Client thread/INFO]: [CHAT] Local game hosted on port 52789
[17:57:48] [User Authenticator #6/INFO]: UUID of player DomosMustache is 972d0492-d7e9-4b30-8fec-29657e8a5c94
[17:57:48] [Server thread/INFO]: DomosMustache[/192.168.1.195:53788] logged in with entity id 62904876 at (-372.0021949684376, 90.0, -1009.5287796189655)
[17:57:48] [Server thread/INFO]: DomosMustache joined the game
[17:57:48] [Client thread/INFO]: [CHAT] DomosMustache joined the game
[17:58:33] [Server thread/INFO]: DomosMustache lost connection: TranslatableComponent{key='disconnect.genericReason', args=[Internal Exception: java.io.IOException: An existing connection was forcibly closed by the remote host], siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[17:58:33] [Server thread/INFO]: DomosMustache left the game
[17:58:33] [Client thread/INFO]: [CHAT] DomosMustache left the game
[17:58:42] [User Authenticator #7/INFO]: UUID of player DomosMustache is 972d0492-d7e9-4b30-8fec-29657e8a5c94
[17:58:42] [Server thread/INFO]: DomosMustache[/192.168.1.195:53794] logged in with entity id 62957975 at (-360.7105311723426, 75.16610926093821, -997.289814050721)
[17:58:42] [Server thread/INFO]: DomosMustache joined the game
[17:58:42] [Client thread/INFO]: [CHAT] DomosMustache joined the game
Error in class 'LibraryLWJGLOpenAL'
    Source '72d21424-9de2-4322-989c-47018f3103a6' not found in method 'play'
Error in class 'LibraryLWJGLOpenAL'
    Source '25233459-e7f1-4659-9e07-f69e657b9f36' not found in method 'play'
Error in class 'LibraryLWJGLOpenAL'
    Source '1f28f044-e65d-44b7-a766-393134e0d52c' not found in method 'play'
Error in class 'LibraryLWJGLOpenAL'
    Source '72d21424-9de2-4322-989c-47018f3103a6' not found in method 'play'
Error in class 'LibraryLWJGLOpenAL'
    Source '25233459-e7f1-4659-9e07-f69e657b9f36' not found in method 'play'
Error in class 'LibraryLWJGLOpenAL'
    Source '1f28f044-e65d-44b7-a766-393134e0d52c' not found in method 'play'
[17:59:21] [Server thread/INFO]: XAcidMystX lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[17:59:21] [Server thread/INFO]: XAcidMystX left the game
[17:59:21] [Server thread/INFO]: Stopping singleplayer server as player logged out
[17:59:21] [Server thread/INFO]: Stopping server
[17:59:21] [Server thread/INFO]: Saving players
[17:59:21] [Server thread/INFO]: Saving worlds
[17:59:21] [Server thread/INFO]: Saving chunks for level 'Summer Of Death'/Overworld
[17:59:21] [Server thread/INFO]: Saving chunks for level 'Summer Of Death'/Nether
[17:59:21] [Server thread/INFO]: Saving chunks for level 'Summer Of Death'/The End

Hopefully this helps. Thanks for reading.