It will be 30 days or more before I can get a UPS, but the UPS is just a band aid. It's not solving the power problem. The same with a bench power supply I could use. It masks the real issue.
My second issue is that everything else in the apartment can handle this "power problem". The only devices that cannot handle it run off of wall warts. RPi5/m.2 is 5v & RockPro64 is 12v.
There is one exception. There is a RPi4 running in the bedroom each day that NEVER has this issue. Not even once. (connected to the same NAS)
I cannot continually monitor the power waiting for a voltage sag. So what I have done is isolate the pi by using an extension cord. I have now tried 3 additional outlets. Each outlet had just the extensioncord/Pi5 plugged into it.
One outlet dropped out within 15 minutes. The other two took a lot longer; 3 to 4 UHD movies played back to back before the interruption.
I know for sure the very last outlet I tried was on a different breaker than the entertainment system. I changed almost every outlet and a few switches that were worn out in this apt. already. So I suspect worn out breakers. The brand of breaker in the circuit breaker box have been out of business for decades. Time to change them anyway (secretly). The apt is 30+ yo.
I'll report back then.
I may try the extension cord method with the RockPro64 if I get time.