My GMC was running great until recently when I was towing a small boat on the highway. The truck sputtered at 60 mph then got worse and finally stopped. The garage who fixed it said the distributer was shot. They replaced it and now the truck has new problems. The oil pressure is much higher and there is a valve knock when under any load or going up a moderate hill, (The engine sounds as if the octane is low) It also stalls out frequently. Other than that the power in the engine is still great. What do you think? The truck was just fine before and had no stalling or knocking of any kind.