There are many possible causes, but a bad solder joint is likely. Touching
up lots and lots of joints might not be a bad idea.

I would suggest taking voltage readings around the PS and recording them.
Then, when the unit stops working, check the voltages.

I realize that it's much easier to troubleshoot when you have the schematic.
But this is the sort of problem that could drive you crazy and waste huge
amounts of time. I would wait until it (appears to) fail altogether.