Sorry I don't know any better way to describe that but attached a photo I took of it.
This happens on my main monitor, typically when playing a game (varies which game). Checking temperatures they seem fine (60-68C range). Card is a XFX Radeon HD 7970 Double D 925MHZ, new, never had this problem previously with my GeForce. Running Catalyst 13.1 on Windows 7 Ultimate 64bit. Both monitors are connected with DVI.
If I play around with screens with this happening I can get the effect to switch to showing on the other screen (monitor #2) which tells me it's definitely not a monitor issue or cabling issue.
Have to reboot the machine to get rid of the issue.
Any suggestions would be great as it's getting quite frustrating.