It's thermal, probably power supply.

Memory errors are reported by memtest as they occur; it executes from
the cache; if errors can't be reported, it means you have a processor
or power problem.

It sounds like a power supply thermal trip; especially as you say it
can't be restarted for many seconds.

Check the power supply fan is rotating properly, fix it if not.  A
strobe LED or torch is handy for that.

Vacuum or use compressed air to remove dust from the power supply.

Disconnect any excess loads within the system box.

If that doesn't fix it, replace the power supply or move the machine
to a freezer.

--=20
James Cameron
http://quozl.linux.org.au/
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