It actually sounds like an overheating issue. When delicate components of a computer get hot, they stop functioning. The most common components that overheat are the processor and the video card. First, open your computer tower and blow all the dust out of the computer. This can be done with a can of compressed air, an air compressor (my personal favorite), or a vacuum on reverse. Be careful not to use a vacuum to suck the dust away, as static electricity can damage the components of the computer.
You should also make sure that your computer isn't in a spot where it can get hot easily. Often, people put computers in cabinets or under a desk, and they can't ventilate properly. On that note, also make sure that nothing is blocking any fans or exhaust ports. This can also impede airflow, which makes the delicate components get hot.
If these steps don't fix your problem, it may be a power supply issue. Have you made any major hardware changes to your computer since you bought it? A bigger video card, more RAM, or other changes can draw more power from the power supply, and the power supply can't keep up with the demand, so it shuts off.