I get it too occasionally. Sounds like a video / RAM issue. When that starts to happen to me I shut down Maya and restart it. What's happening is that all the memory cache being stored by Maya is building up more and more and starts to bog down the system. By restarting Maya you clear the cache and free up needed memory.

But then it could be those little green things I found clinging to the bottom of the video card. Damn things bleed when you poke them with a needle. eww!