I have been playing far cry 3 on steam. it is a really great game but every 20 minutes or so it shuts off. this is very nerve-racking and annoying. someone please tell me how to fix this. my computer is an acer quad core with windows 7.
That sounds like a heat related issue based on the few details you give. So just as a quick test, pull the side off the computer and try playing the game like that. If the duration you're able to play increases noticeably, you have a heat problem.
It could be a heating problem, change the nano grease, clean the fan. Check the bios info for the fan rpm, maybe it spins too slow.
It's less likely that it is a PSU problem. Do you get random shut offs when doing anything else?
Far Cry 3 has a tendency to heat up cards more than other games. Its like how a benchmark heats cards much higher than what games do. It just stresses it more. I've noted hitting close to 90 degrees for one of my GPU's in FC3, but in other games I never surpass 75.