PC restart while playing game

Hi everyone,

I am having a new problem with my computer lately (I don't have the specs with me right now) : When I play an older game (Day of defeat : Source) it randomly just shut down and reboot. No blue screen, no error report. If I start playing again, it will shut down within 5 min. If I let it cool it down, I will have more time to play. The times it takes for the first shutdown is never the same, could be 30 min or 2 hours ! BUT when I play a more recent game (Overwatch) I can play many hours in a row without any problem !

I didn't overclocked anything, just to be sure, I went into BIOS and set everything to default settings with no change. I cleaned with air can the computer, I installed something to see if it was heating up (don't look like it is). I saw some suggestions that it may be my power supply ? Any idea guys ?

Thanks !