I have a Chevy Silverado 2500 with the 6.0L engine and it has been idling rough. When I put it in gear the RPMs start fluctuating and the truck wants to stall out. Once the truck is running down the road, the RPMs smooth out and it runs fine. I put a fuel pressure tester on it to find that once the engine starts stalling the fuel pressure gauge bounces back and forth really fast. There are no engine codes. What could cause these symptoms?