1989 Chevy Truck Oil Pressure issues

My 1989 Chevy PU K10/K1500 4WD will not hold oil pressure once the engine is warm. If i accelerate the oil pressure will rise but very limited power and water temperature climbs to 210 very quickly. When I come to a stop the oil pressure drops back down to zero and my check gauges light comes on. Should I be looking into replacing the oil pump or sending units or does this sound more like a lower bearing issue. PLEASE HELP I live out in the country and getting the truck into a mechanic is an issue. And yes the oil levels are accurate, changed oil and filter this morning and even added a heavier oil than whats recommended 5w/40 They had a lot of problems with the sending units back then, might be worth a try?Car Repair Talk's forum.