I'd lean towards the PSU too, you could test the PSU with a multimeter
This link describes the procedure well
personally though I tend to take out the PSU jump the black and green pins of the 20/24 connector to turn the PSU on then test each molex to see what outputs I get.
I know a lot of people say that you cannot test a psu that way as it is not under load, but at least it gives you some indication.
You are probably more knowledgeable than I am but hopefully the above would be of some help :) Good luck