jerky movement

Using a completely fresh unaltered version of Minecraft and the latest snapshot I then proceeded to create a new world. I spawned under a tree with my avatar partly in the ground and jerking up and down. I was able to jump out easily but I noticed every now and then my movement was jerky as I proceeded to punch wood, gather food etc. I spawned in a rather large Tiaga biome and didn't want to settle there so I set of exploring. I was skipping over an icy lake and came to another section of the tiaga biome. I fell into some water and splashed my way to shore but it took me several attempts to jump out of the water. After I finally got out of the water I proceeded to kill some chickens but my movement became increasingly jerky and I was unable to move after only a minute or so. I was able to replicate this jerky movement phenomenon by doing the following things: Creating a completely fresh minecraft - re-downloading all files; updating it to the new snapshot and creating a new world. At first I was able to go a full minecraft day without anything bad happening other than some very strange graphical glitches (( more on that later )) - My spawn was a lot smoother the second time - no jerkiness. However after exploring my world a little bit; gathering food etc I came across a NPC village just after the sun went down. I was being chased by several spiders and intended to reach the relative "safety" of the village. I jumped onto one of their farms but once more my avatar started jerking about and I was unable to move. It took the spider to knock my off the farm but after killing it I was still unable to move at all - my avatar jerking up and down like crazy. On a side note: Generated structures were appearing in my world with parts of them completely invisible. See attached file for example. I came accross a desert temple; a desert village and a regular village - as well as some cacti - that were all invisible. Loading and unloading the world worked for the invisible walls but wouldn't work for the jerky movement.