I would first test the RAMs by using two at a time. If it runs fine with one particular configuration, then you've nailed the problem.
My guess, though, is that the motherboard is giving up the ghost. Would suggest bringing it down to the motherboard manufacturer who will test it and exchange or repair it if under warranty.
CPU's seldom fail.
The other possibility is the power supply, but you can easily test this by swapping boards or casings.