Chances are things are wearing out. If the engine is original, a new pump wouldn't hurt. When you're stopped and put the truck in gear, you're lowering the rpm's thus slowing the oil pump. If it is worn enough, it won't supply enough pressure until the vehicle is put in neutral (rpm's slightly increase) or you hit the gas and they increase. Thus, more rpm's pump speeds up and increases pressure.
It does sound like it needs replaced.
NOTE: You can have the oil pressure checked to see if it's too low. Then you could make a more educated decision.
If you have any other questions, let me know.
Friday, October 10th, 2008 AT 11:24 PM