I have a 2004 silverado 6.0 liter 2500hd with 65,000 miles. Lately after its been sitting for say 8hrs I'll start and warm it up. When I drive away it has no power for about 100 yards then it goes back to normal. I had to put the gas to the floor to pull my boat out of the water and it would barely move. Then when traveling down the road over 45 mph unloaded it will sometimes stumble.