Quad Q6600 2.4
Nforce 790i Sli
Epsilon 800W PSU
built computer myself, computer has been running more or less perfectly for over 3 yrs
Computer randomly shut down twice, now whenever I press the power all the fans (psu fan aswell) start and then it shuts down 1-3 seconds later, no signal being received by the screen.
Removed everything and tried powering up with just the board, didnt work, tried with 1 stick of ram and different variations of ram, didnt work.
I thought it might have been the CPU overheating at 1st as the mobo 'hot cpu' light was on for awhile, so removed and reapplied thermal paste. Still no good, i figure its the PSU, the mobo or the cpu, any ideas?
Most common causes of this would be the PSU or else the motherboard shorting to the case somewhere. Since you have a rather ... questionable brand of PSU, I'd check that first. If you have a multimeter, try measuring the voltages on your PSU and see if they're within +/- 5% of spec. Alternatively, if you have access to another PSU that you can swap in, try that in our machine and see if it improves things.
If this still doesn't help, try taking everything out of the case, put the motherboard on a non-conducting surface with a bare minimum of components and see if you can power on that way. That would be an indication of the case shorting problem.
For reference: If it's a heat problem, what ends to happen is the machine will shut down unexpectedly, then if you try to turn it on immediately it'll do the start-then-stop that you were describing ... but if you let it cool off for a while, it'll work normally again (until it overheats again).