I've been having this problem for quite a while and I'd convinced myself it was an incompatibility issue with my AMD graphics cards. However, I just upgraded to some newer nVidia ones last night and the problem is back.

The mouseover preview windows, alt-tab, and even mouseover tooltips for UI elements just blink onto the screen REALLY quickly and vanish. I have to use Windows Key-Tab to cycle between windows. I can get this to go away for a while by rebooting, but it always comes back. I'm guessing this problem isn't unique to me - anyone out there able to help/point me at the thread where this has been solved?

Certainly. The next time the bug manifests itself, I'll test it.