My computer runs ok for about 10 minutes then turns off?

my computer cuts out after about 10 minutes of using it. I've changed the mother board as I had a booting up problem, there were two damaged capacitors visible. It now boots up ok but still turns off after about 10 minutes. I've changed the memory and power supply, also I've disconnected the speakers, re-boot Switch and a couple of USB's. Also I've connected the power on circuit to the re-boot switch to eliminate the power on switch. Any ideas?