i need help and im not getting any my minecraft is not good pls help

everytime i create a new world after a while i have problems with :
1. the game crashes and you saves cannot be re used
2. the game creates a large black void which is about 50 blocks x 50 blocks that starts from bedrock and goes up to who knows if a mob gets caught in it it falls then repawns and falls again over and over if you go near it it kills you if you smash the blocks its on it freezer the game or crashes it when you walk near it the game slows right down.
3. i play a new game and i get ramdom freezing so when it stops im normaly dead and lost everything.
plz help me this is getting stupid i want to play the game but its getting annoying
many thanks andy