Computer shuts down automatically

My computer shuts down by it self all the time. I can get into windows most of the time without it completely shutting off, but after about 5 min (sometimes it can stay on for upwards of 2 hours) it shuts off. The only way to restart it is to switch plugs or flick the power supply switch in the back. Im thinking a faulty PS is at fault because i have flat caps on the motherboard and the fans seem to be working fine.