Having trouble with my ASUS K8V-MX

Computer works fine for hours then suddenly bluescreens.
When I reboot, it sometimes works fine, but usually everything starts
up (fans, leds etc, I can hear the HD spinning up and the HD led stays
on), but there is no "click" and the BIOS screen does not appear.

If I wait 20 minutes or so with the power unplugged, it will boot
normally. If I leave the power plugged in, it will not.

Memtest normal, HD surface scan OK. Temperatures are normal as are the
voltages, according to HWMonitor.

What is the audible "click" I hear just before the BIOS screen, and
which does not happen when it refuses to boot ? I can't locate it.

Is it something on the MB or is it a PS "noise" ? I swapped out video
cards, nothing changed.
TIA
[]'s

PS My wife's Desktop, same MB but a later model is having exactly the
same issues ...
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