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#21
Very good troubleshooting so far.
When you did the test as indicated in your first paragraph, did you short between the green and black wires on the 20 or 24-pin connector?
Plug components back in - one at a time - to see if a component may be causing a short?
Did you test another PSU in your system?
You mentioned it but didn't see a result.
As mentioned before, a 5 volt reading doesn't mean everything is good with the PSU. A low voltage reading does mean there is a problem, then there is the intermittent faults that can drive you crazy when trouble shooting.
Do you have a voltage regulator, UPS or surge protector in line?
Power outages can effect the above items as well as your computer.