I own a 1998 K3500 Dually with the 6.5L diesel engine. I have been unable to diagnose this problem, so the vehicle is getting no use for the last year. When you start the engine, it goes into a very fast (1500 rpm), rough idle. Kind of oscillating. It will keep going like that even after warming up. The moment you shift into drive or reverse, it goes back to idle and behaves normally, until you press the accelerator pedal, where it accelerates severely. It doesn