Before you spend shitloads of money:
Clean your PC from the inside (Works perfectly fine with a vacuum cleaner, turn it right down and carefully clean it)
Then what you also wanna try is take everything out of the case and put everything back in.
I've had the same problem before. Turned out I somehow shortened my PC. Didn't touch the PC to turn it off, but it still occured.
So taking everything out and putting it back in is to make sure that there is no short-circuit.
Simply checking whether the RAM might have loosened itself a bit or something is worth a try as well.
Try using a different power cable as well. These are fairly cheap and if you buy a new one for nothing it's not gonna hurt your wallet really.