Random Shut Downs

I have an Inspiron 546 running Windows 7 with a AMD Athlon II X2 240 Processor 2.80 GHz with 2 gigs of ram and a 64 bit OS. I have had it for a few years now, but within the last year it has been spontaneously shutting down at certain times. It has only happened at times when I am playing some kind of video game. I asked a friend of mine who is a a little better with computers than me, and he unscrewed the side panel of the CPU and the inside had a good amount of dust in it. After cleaning it, the computer ran fine and it stopped giving me that problem. About a day ago it started again, I was playing a game and it shut off. I opened it up and cleaned some dust out, and it seemed to work, but today it did it again. I went on the dell website and ran a diagnostic, but all it said was I needed an upgrade for the DVD-ROM drive. I'm at a loss, luckily the computer works fine when I'm not playing a game on it, but I would like to know if I can fix it somehow.