i have an 02 2500HD, allison trany, 8.1 gas engine. when i am at 55mph or in a slight incline in OD the truck seems to shudder or slip. and now since i have let it go for about a year, in the lower gears it will happen as i accelerate to. I have had a Kat Mech ride with me and he says its not the trany. could it be dirty fuel filter or throtle position sensor??? HELP!! I'm so sick of the slipping feeling!!