Editor breaks geometrics in viewport and other issues

First of all this is my first post here on the Unreal Engine forums, so Hi all. I’m new to the 3d game development world and I haven’t worked on something like this since I was younger so I have afew questions…

a) Sometimes when I’m moving blocks around my view port, Another block will become invisible or partially invisible… Why does this happen? I have a floor in a level I’m designing that is almost completely invisible… (I can provide pictures)
b) Everytime I Undo an action, the editor locks up (both Blueprint window and viewport window) Any ideas why?? (I have an FX8350, GTX 770 and 16Gb of mem…

I might have afew more questions later, once I get through the animation videos etc.

Thanks guys!