Low Hot Oil Pressure E30 325is

What would cause my oil pressure to drop after 5-10 minutes of hard driving? If I stop the engine and the restart 1-2 minutes later, the pressure is fine. I can think of two things: Bad VDO sending unit or oil pressure relief valve sticking open. I replaced VDO electrict sending unit and gauge with a AutoMeter mechanical gauge, I inspected the relief valve and it appears to be fine. Oil Pump is new and has good pressure when oil is cold or warm. I use Monil 1 Full Syn.