This seems to only be happeneing once the truck is warm. My Tach only works once i idle the engine to 1500 to begin with but while driving in town when i first noticed. Wether the od drive button is on or off. at about 35 to 45 if you don't have a hard accel on the pedal getting up to speed on the high way. While under light accel the trans doesnt feel like if shift just the engine goes from 1500 and falls abruptly then back up until on a hard accel or giving it more pedal. I don't know what you would consider this but any idea where to look replaced return lines. Haven't changed the fuel filter any help would be appreciated. I drive this truck everyday and put 100 miles on it this month. It's a 91 7.3 idi with a turbo 4 wheel drive.