Whenever I play Minecraft on my iMac, it freezes up and when I exit out of the game and go back into it, parts of the Woodland Mansion are missing. This only happens on most worlds but some don't do it, while some will only do it if I leave my small town.

Whenever I play Minecraft on my Apple iMac computer, Minecraft will freeze for about a second then unfreeze for about a second and this will go on and on until I exit out of the whole game. When I go back into the game, parts of my Woodland Mansion will literally be gone so it looks like it was never there, because on the ground where it should have been, there will be trees and stuff. This freezing thing will only happen on certain worlds, while some of those "certain worlds" it will start to freeze again if I leave my town. This has been going on since around June of last summer, 2015. I have no idea if this is a glitch, a bug or if it is a virus on either my computer or Minecraft itself. Whatever, please fix it as fast as you can, because it gets extremely annoying and then I am not able to play Minecraft.