I've hit that bug fairly often since I moved to Vista x64.
I have observed it in a test session on my win2k8 server too.
From what I could see it seems to be linked with two type of conditions:
1.ping/pong events(regardless of if they should be displayed or not)
2.gdi redraw.(though that one might just be a consequence of #1)
(the issue seems to arise a lot more on vista than xp, which let's me think there's a part of gdi issues in it, since vista's gdi is about 3 times slower than xp's(for aero&architecture reasons, there's no more gdi hardware acceleration hooks in vista)
