buddy what about ur cooling system.
It can happen when ur PC becomes hot.
i have have discovered the culprit:) it is the PSU. It only delivers 26A on total over the 12V leads which means it's underpowering my gtx460 (24A minimum recommended by Gigabyte). It's really funny, in the morning when system is cool and PSU cooler is not so loud, the games work fine then gradually decrease in FPS to 20-25 and it's unplayable. i also noticed if i undervolted the CPU it helped a bit.
ordered a new OCZ 600W Slient extreme 2. Should do the job.