Could you please explain what you mean by "I replaced the motherboard, processor and memory" ?
Did you buy new hardware or just dismantled the PC and put things back in ?
My answer below is based on the assumption you purchased new parts.
I'm guessing by changing all those parts, there shouldn't be any dirt clogging your Heat Sink or Case Fans.
Obviously you have changed every possible thing without any success so you might as well do the PSU to finish it up.
If nothing changes after replacing the PSU, we'll move on to other areas then.
This is a classic scenario where I would have started with PSU.