Oil pressure gauge?

I have a 1995 Pontiac Transport mini-van with 190,000 miles on it and I have an oil pressure gauge problem. It bounces from normal to pegged but lately, it is pegged constantly. I have replaced two oil sending units and neither of them fixed the problem. The car runs ok, but sometimes it will buck during acceleration but I’m not sure its related to the oil gauge issue. I have also noticed that when the gauge peaks out, the AC runs hot. When the gauge returns to normal, the AC temp cools down again so they are definitely related somehow. Repair shops have told me that if it isn’t the sending unit, it can only be the oil pump, but wouldn’t I know if it’s the oil pump? One suggested I contact a Pontiac dealer to see if there has been any recall for this problem. Engine temp is fine and seems unaffected by the peaking oil pressure gauge. I noticed this problem over a month ago and have put nearly 3500 miles on the car since I first noticed the problem. I change the oil every 3000 miles and just changed it three weeks ago. The problem still persists. Any thoughts?