I have the same model (albeit with a Sonnet upgrade). The RAM slots are known to be a bit picky / quirky, one in particular but I can't recall which one. (I got to know after mine died some years ago and found others online with the same problem)
Other than that, i found it to be very reliable and a true workhorse. That is, until ... my PSU died last year! Suddenly my MDD started to act weird. Sudden total freezes (no MacsBug) to outright powering off the machine in mid-use.
If you say you checked the PSU and it gives odd numbers my best bet would be looking for a PSU replacement. After I replaced mine with a spare one I had it booting up and working fine ever since! Given these PSUs push towards their 20 years of service, you'll eventually have to replace it anyway. At least that is how I see it ...