What caused this?

[F. S. Kuhl]

I took a 3/140 to another facility for a demonstration.  When I powered it
on there, it refused to boot.  All 8 of the LEDs would flash together at a
rate about twice a second.  It sounded like a relay in the power supply
was clicking in synchronism with the LEDs.  The monochrome monitor, which
draws its AC from the cpu box, appeared to have power applied to it
intermittently at the same rate.  When I brought the corpse home, it
booted and ran perfectly!  I can't get it to fail.  Neither could the Sun
tech who looked it over.  It occurred to me that the AC voltage might have
been out of range, but there were several PCs and an Apollo ('scuse my
mentioning them) in same room later working perfectly.  Any ideas?