2003 Hummer H2 03' H2 Hummer oil pressure slowly dropping

Hi, I have an 03' H2 Hummer with oil pressure prob. I recently replaced the oil pressure sensor and did not take care of the prob. The pressure will only drop when driving for an hour or so. I drove the car on the freeway for about 90 miles till I noticed my oil pressure gauge is at zero psi. I still drove another 10-15 miles before stopping and checking the oil level, but it was ok. No signs of engine knocking, vibrating, loosing power or even engine light or oil warning in my dash. When this prob happens and I shutt the engine off and wait for few minutes, the gauge would come back at a normal reading at 38-40psi at idle, but if you shut off the engine and start it right away, the gauge gets stuck to zero psi till it cools down a bit. Please help!