the last two posts mentioned video card rebooting. It could be a driver issue. Always make sure you have the latest card drivers installed. If you have an NVidia card, the new drives can sometimes cause more problems than they solve. If you have more problems after installing the new drivers, rollback to an older version you believe was stable. I have had to do that a few times. The newest NVidia drivers seem much more stable so give them a try.
Also, keep an eye on the programs you run in the background. they can put additional load on your CPU. I saw if they don't have to be open, go ahead and close them to minimize this issue. You may also want to give your comp and nice good cleaning with some compressed air if you haven't done so. All that dust can really drive up your CPU and GPU temps. If they get too hot, they can often shutdown on their own to prevent any damage to them. If you have your computer sitting under a desk, I would recommend moving it to a place with better ventilation. You have to let your comp breath or it will die an early death.