Oil pressure problem

Ok guys, I know what pressures the truck is supposed to run at so this isn't going to be one of those threads. My friend's father has an LBZ. He asked me if I knew and I couldn't give an answer so I came to you guys. When he is driving down the highway and stops for a toll the guage drops to 0, the only way to bring pressure back up is to put it in neutral and rev it up. It also does this around town at stop lights and such. It is still under warrante and at the dealer now but they called him and told him that they can't get the truck to do this so they can't diagnose the problem. You guys have any ideas? This is all the info my friend really told me so if you need more just ask and I'll get the info for you. Thanks,
Victor