VAX 4000-300 shuts down

All, I'm wondering if someone can confirm my diagnosis for a problem with my VAX 4000-300. One day, after it had been running for the better part of a week, I found it unresponsive with only the AC light glowing. I suspected an overheat problem, but the overtemp light wasn't on. Nonetheless, I shut the machine down, left it for a while and came back. It started up again, booted into VMS and seemed fine. I let it run for a half-hour or so, then shut it down normally. That was two days ago. I just tried to power it up again to do some work, and the power-on tests fail about halfway through (but never in the same spot, so I don't think it's actually a power-on test failure). The machine goes to the same state each time: just the AC lamp glowing. I even have the fans on high (and that's noisy), but no go. I figure it's time to replace the power supply unit (or refurbish it, but replacements are still relatively cheap). Any other thoughts? Thanks -- Ian