Without any actual system info this isn't going anywhere, but since this is some sort of Mac, I'm simply willing to write it off as one bug (in OS X) on top of another bug (in AE). At the very least you should check the relevant screen settings system panel and play around. Using excessively large resolutions may exhaust graphics resources as may using virtual desktops or multiple scrfeens. And then there's of course all sorts of things that could be going on if you use video conferencing tools, instant messengers, remote desktops, screen recording tools or whatever. Point in case: Reduce the burden as much as possible and try again.