Now one last suggestion is that you check very carefully (as you can) the joints, caps, and other components for any sign of loose fitting cards stcikc etc, dry joints - that is to say any components that look to be unattached properly to the motherboard, and bulging, leaking and/or burnt caps - that goes for any scorching on the motherboard itself (it will smell bad) .
A very tedious process and some of the caps may be hidden under the cooler plate for the CPU and some VERY small.
Edit Don't forget to check out any port sockets too ie USB