Everything is always crashing! And no one can fix it?

My pc, is getting me crazy. It has something really wrong with it, that even the guys from the store cant fix it. My video games are always crashing. And not only the video games. Sometimes my browser or my messenger just don't work, and I have to reboot, and the pc crashes like 1 time every 2 days. But the most annoying thing are the games crashing. I has been happening for months and I sent it to be fixed 2 times already. The first time they checked the software, graphic drivers, rom, ram etc, and they told me that if it continued to happens, that the problem was the power supply. It continued, so today they changed the power supply, but the games are still crashing and freezing. I don't know what to do. I just want to smash the computer. Any idea what the problem might be?