After upgrade to OS X Mountain Lion a Virtual Machine in Fullscreen mode flickers frequently. This issue occurs only if a second display connected to your Mac computer
CauseThis OS X issue occurs on ATI 6xxx video card on OS X Mountain Lion only.
SolutionDownload and install the latest OS X 10.8.1 update: click on Apple logo > Software update... and install the update
If the issue still persistsDisconnect the second display from Mac
Workaround 2Disable 3D acceleration in your Virtual Machine:
- Virtual Machine menu > Configure... > Hardware > Video > check off Enable 3D acceleration