PC occasionally freezing

I recently bought a new motherboard for my main PC and shunted others down the line
until I got rid of an old one causing problems. The motherboard originally in my
main machine (ASRock Fatal1ty H370 Performance) went into the machine in my bedroom
(replaced H270 Performance). But I've been getting occasional freezing on that since
the change. Everything locks up and I have to hit the case reset button to reboot.
Once Windows restarts, it runs with no apparent problems. The event viewer indicates
ID 41, Kernel-Power, so I put a new PSU in. No change. Still getting freezing that
occasionally occurs in the first 20 or 30 minutes the machine is on, never after that
so far. When it happens, it tends to be either during or immediately after a
procedure such as running a Windows Defender scan. Yet nothing is overheating. The
board has the same CPU (including heatsink & fan) and memory modules it had when it
was in my main PC and ran with no problems. Incidentally, the new and original PSUs
check out fine and both run other machines flawlessly.

Any ideas what could be causing these freezes other than PSU or overheating? I'm
thinking an electrical connection on the motherboard could be grounding out on
occasion before everything heats up properly, but that's really just a stab in the
dark considering what happens and when.

Larc