Ok my friend has a 92 250 Cummins with and auto which I believe is the A518 transmision. Anyways it doesnt like to shift into overdrive between 45 and 55 mph, to make it shift you must let up off the throttle for about a second and slowly lay into it. and if you are in overdrive below 55 mph and give it barley any more fuel to accelerate it pops back out of overdrive untill you are going faster the 55. it seems to like 65-70 mph range. What is the problem. Is it a sensor or some kind of linkage or other?. Also the only "mods" on the truck is straight pipe and a k&n filter with a swiss cheezed airbox. It has also brought the milage down a couple mpgs.

 

Check/adjust the TPS first. It can be cleaned with electrical contact cleaner and some light compressed air if it's giving off some erratic readings. Also check to make sure the cable from the transmission stays in the forward position, and is adjusted. Mine required a generic spring to apply some forward pressure to keep it in place. TPS's are expensive for what you get, there are some great idea's in other posts to eliminate it too, do a search on TPS.