i'm trying to recap according to the OP
Gauge, he says when it loses pressure it won't even pump oil out the pressure port.
Bearings, if something is a problem bearing wise any engine that would go from 80# to 0 is going to show it some other way.
Pump drive, said it's been replaced
Pump, it's been replaced and they've been disassembled and inspected
Filter, been replaced
Lifters, if it was uncovering the galley it shouldn't come and go. if it had a bent or displaced pushrod the engine should show a problem by being noisy or running rough
though i've never seen it happen i guess it could be possible the inner rotor is spinning on the drive stem but OP says the pumps have been inspected and if this was happening you'd think it might show metal in the oil although anything from that should go through the filter
now just to make sure i'm understand you correctly it not only loses pressure intermittently on a restart but also when it's been running it will lose pressure while running?
starting to sound like you need to call Father Merrin