Hi guys. I'm new here

I have a 1990 S10 Blazer. It has a newer Vortec 4.3l in it... not sure the year.

Anyway, it's developed a strange issue. After driving for a while with the engine warm the oil pressure gauge will climb all the way up when I'm sitting still at a light or in traffic. Once I'm moving it goes back to normal. It will also drop to normal of if I turn off the a/c or pop the truck in neutral or park. Pressure stays constant at warm up and it hasn't blown out any oil filter o-rings. Truck runs great with no leaks or weird noises. I've changed the filter twice just to experiment.

I haven't checked it with a mechanical gauge... don't really have the means to do so.

I'm stumped.