MADe, you're perfectly right. That's exactly what was suggested in the first answers to the OP: Backup and HDD diagnostics, if no results then RAM testing, then PSU, in that order.
Since there hasn't been any response from the OP, a natural assumption is to suppose the problem hasn't been solved and therefore the discussion starts to spread beyond the problem in question.
@Chalenge, yes, and alongside of ripple I'm interested in finding any anomalies in a PSU which is starting to fail but hasn't given the big bang yet. I understand that a $15 PSU tester is just a dedicated voltage meter and not any better or worse than a multimeter or the readings in BIOS/windows voltage indicators. Neither of those can tell if the voltages quadruplicate for a split second, the refresh times being long enough to be read. In my wish list there's a pocket size PSU tester which could display all possible ways a PSU could fail in a minute or two.