I once had a power supply that would run fine for hours or even a day or two. Then, suddenly, the computer would boot. Every time I checked the supply, it checked out okay.
I connected a volt meter to the 12v side of the supply and connected its PC interface to the computer. A program on the computer records the voltage every second, five seconds, ten seconds, minute--whatever I choose. I set it to every second.
I had another of the same meters connected to the wall socket, also recording every second.
After about 30 hours, the computer booted again. I checked the voltage logs and the wall voltage was right on; but, the power supply's 12v side had dropped to somewhere between 9- and 10 volts. (I forgot the exact voltage; it was about three or four years ago.)
Anyway, testers are nice and I wouldn't be without two in the shop, one in my office and two on the service van; but, I hope I never run into another like the one several years ago. A simple tester simply misled me on that one.