Thanks everyone for the tips. I purchased an extra gauge and need to hook it up. I'd dismissed the gauge as a possibility as I didn't think a bad gauge could cause such odd behavior.
In regards to bearing clearances, I didn't make it clear but the pressure was rock solid at 40 psi for the first 10 hours then suddenly started acting up.
Oil pump sucking air is an interesting idea. I don't remember any O-rings or other sealing devices on the oil pump inlet. Need to look at the manual again.