I have a 04 3500 4x4 5.9 auto, the truck at 60 to 75 mph when you maintain a constant speed will start to slow down about 2mph and then resume your set speed, this is with or without the cruise set, it will accelerate with no hesitation ,other than the fluxuation of speed at a set speed the truck runs great. I have changed fuel filter and air cleaner , any help would be appreciated thanks

 

dont know about the 04s but my 06 has the fly by wire throttle. (no throttle cable) Could be the pedal potentiometer

 

Have any power adders? Power adders can make the throttle more sensitive to inputs.

MikeyB