Blank spots on my world/also logs me off alot

I have been building a huge city for about a couple months and then I've been noticing blank spots in the ground that stay there and mess up anything in that area, also the sky has been turning in blank spots but those eventually disappear (the sky one does not need fixed as much as the ground one.) Also I have saved this world and I don't know if this will effect it.
Another small bug I noticed is when you place on concrete or wools or glass it will log you off after you place on like six or so. (Please send an email I can respond to.)