Bad Crashes and In-Game Problems

Crashes
 Randomly crashes for no reason. Mostly crashes on startup. Sometimes you'll be in the game and if you try to open anything it loads forever and then it says an error happened.

 

In-Game Problems 

 Don't know why but for some reason my tamed mobs randomly disappear at different times, and also go invisible. While playing some blocks also go invisible and then you can see through blocks and you don't know where everything is.

 

Plz fix this