Sorry, I wasn't clear. I meant that the PSU ran fine when I jumped it, not the system. Running the system "barebones" out of the case has the same no power problem.
My self built computer, which had been running fine for couple of years, started random restarting a week ago. I assumed it was software related issue and updated all the various drivers, patches etc. I also updated the motherboard BIOS. None of this improved matters, and the restarting...