Chris alluded to the problems possibly being in the display drivers, and I have to say I also believe that to be the case, but I wouldn't guess it's a bug so much as a resource shortage, given that it's not persistent, didn't cause a crash, and is not reliably reproducible. I can say with certainty that yesterday, when I was seeing this problem, I had a LOT of windows open doing a bunch of different things (I am an extreme multitasker) - including a fair number of Internet Explorer 9 windows, which we know tax the GPU as well. Further, some of those windows, such as VMware Workstation, were themselves resource hogs, in addition to Photoshop.