Dead motherboard?

Symptoms: started rebooting occasionally, progressively increasing with prior mouse
pointer fadeout or freeze (cordless and corded mouse) - stopped seeing USB wifi
adapter, although USB drives would still work - now won't post at all (power comes on
and case PLED and fans run for about 20 seconds, then reboot, rinse and repeat) - no
HDD LED - no signal to wake monitor - power to DVD burner on. Multiple reconnections
and even reseating of the CPU and heatsink/fan and trying different memory modules
didn't help.

PSU: Tested thoroughly with comprehensive digital PSU tester and everything 100%

Is there anything else I should check out? I'd like to learn it's not the
motherboard since a replacement that could accommodate the 7th generation i3 CPU in
it would cost about as much as a new 300 series board and i3-9100 CPU combined.

I'm not running a higher powered setup here since this isn't a main system, but one I
use for Windows Insider development testing.

Motherboard: ASRock Fatal1ty H270 Performance
PSU: Corsair CX500M

Larc