Strange problem, maybe hd4850 / ATI but not sure

Hello,

I purchased the game Portal a month or so back, and since then randomly during game play, everything will freeze, then my computer will restart without warning giving out a long beep like its in pain or something. It will then attempt to reboot itself but it will fail. I have to manually turn it off and on to get it to boot. Then when it gets onto the vista desktop, vista's sidebar will have lots of blue pixelated lines running horizontally through it, the severity of this is different each time, they could be right across the screen or they could only be 1/4 of the way across. I then fix this by suspending and then bringing it back and everything is fine (I also assume restarting again would fix this also). Its all very odd and confusing, during normal running its absolutely fine, runs without a hitch.

I recently applied the latest gta iv patch to see if it would fix the game issues that caused me to give up on the game 4 or 5 months ago and it did. But last night it did the exact same thing as it does in the game Portal. Although I can play games like LFS and RBR for hours without any problems.

Has anyone got any suggestions?

My specs are

Intel Q6600 @ 2.4Ghz
4GB ram
HD4850 running one monitor at 1920x1200 and another at 1280x1024
Gigabyte GA-P35-S3G
Zalman 500w modular PSU

Catalyst version 9.5
Vista Business 64-bit