oil pressure gauge problem

I have a 97 jeep cherokee 4.0
When I purchased it a few months ago the oil gauge didn't work. It kind of floats around and after about 20 mph drops to zero. The PO said it was the oil sending unit. I sense have replaced that but the issue continues. Any other ideas on what could be wrong?
Thanks

If the sending unit is bad, it will peg out to 80.

If you have driven it for a month I would guess that you do in fact have oil pressure and there is a problem either with the gauge or some wiring issue.

Could be a bad sending unit, gauge, or wiring in between.

I hear that dealer sending units work best. I put a NAPA Echlin on my 01 XJ and it works great. When I got my 97 XJ that was reading low I put a NAPA Echlin on it and it works better, but still does not show what the mechanical gauge does. I'll either try another NAPA sending unit or buy one from the dealer. Since the wiring is different between the 97 and 01 I can't easily swap parts and I'm not sure how to test the gauge yet.

A gauge reading 80 is usually a bad sending unit, or if just replaced, the wrong sending unit, but the sending units also fail reading low.

Get a mechanical pressure gauge and test the oil pressure.