Strangely enough my Jeep has the same issue. I just purchased the jeep a few months ago, and the oil pressure gauge, if it moved at all, just fluttered around aimlessly. As part of the sale, the PO gave me a new-in-box sending unit.
I replaced the sender this weekend, and now the guage sits right at 80psi whether the engine is idling or revved up. I assumed I just got the wrong sending unit, and it's not 'calibrated' to work with my gauge. The good news is that I know I have oil pressure now ;-)
(I'm running stock 304 V8 with stock gauges, the jeep is a 76 CJ7)