2000 Dodge Durango what else could make the oil pressure dr

I was wondering what else could make the oil pressure drop.Ihave changed the oil pump,cleaned out the oil pan,cleaned out the intake and got all the sludge and build up out of the motor. the oil pressure still drops to 0 pressure. It happens the most when i would drive 60 to 65 mph. Please help me out.
Misty Mcmillan