Just with css or does this also occur with other games like if you played HL2 for a long duration. The first thing you should do is check you temperatures. Use core temp and gpuz to determine you cpu and gpu temperatures. Once you confirm that nothing is overheating you next need to ensure that you are running the latest drivers. If none of these are the problem than it is your power supply. I would actually guess that first but that is only because most people use cheep psus that are underpowered and they generally produce that result. However I would test the other options first to confirm. In the mean time what psu are you using?