2002 Bravada w/120,000 miles...all good until recent oil pressure problem. Starts OK and drives awhile locally w/no pressure problem after warm-up. Eventually, when stopped @ idle (600 RPM) the oil gauge pressure drops off fast and "check gauges" light comes on followed by "Low Oil Pressure" readout and then "Stop Engine". If I give it a little throttle (to 700-800 RPM) the pressure comes back to 45-50. I had the oil pressure sensor replaced, no oil burning or leaks. Had mechanic look at it and he changed the oil and added an oil treatment. Has made no difference. It does not knock when pressure drops off although I do not let it stay there. It does shut down hard when I'm done driving. Recently, I noticed that when I try to idle at 700-800 RPM the idle lopes. I've read that it could be the oil pickup, oil pump, main bearings...etc. But it makes no sense to me as I've driven the SUV for 20-30 min in local traffic and it's fine. Due to the loping idle I added fuel injector cleaner and cleaned the throttle body. I am totally frustrated and no one can give me an answer. I do not believe the main bearings are shot as I religiously change the oil and never muscle the car. If it was the bearings it would have a more consistent pressure problem. After all, the oil is hot after 10 minutes and does not lose its viscosity. HELP!