I have a stock 2001 2500 Duramax. It has a Pac Brake exhaust brake. I pull a 5th wheel trailer and going up a long fairly steep hill at 2500rpm the motor goes into limp mode and the check engine light comes on. The limp mode last for awhile then is ok. After a few starts the check engine light goes out. The truck is not working hard at all when this happens. The second time it happened it cleared up and the truck was doing 3000rpm up a pretty steep hill and nothing happened. The light was still on. Any help would be great. Thanks, Scott