I think its problem for PSU. I had Same issues 2 years ago as old pc.In Old pc as PSU, I did disassemble PSU and to see it has some components are Damaged due to Short Circuit of power. So can you try to disassemble for old one PSU? I know it is not recommended But at least you should know it what happened to PSU and your PSU is old and warranty are expired, right? try it chance If you are confident, open up. if not, then go to computer service to check up.