I know jack diddly squat about Mac Pros - although it's a beautiful machine. I will say, however, that I had several PSUs go on me back when I had windows machines - happens all the time. Did you check with a simple voltmeter to see if you're getting the expected voltages from the PSU? Again - I don't know about Mac Pros specifically - but my old Windows boxes I could just pull the PSU - and check for the expected voltages. I would imagine you could do the same in this case to validate your concern - just as a next step?