I have a 1993 Isuzu NPR. All of a sudden it quit shifting into overdrive. While in drive at 35mph or so, if you stomp it (not necessarily to the floor) it will shift into overdrive, however, if you lift off the accelerator or the truck levels out based on where you have the accelerator, it immediately drops back down into drive even if you are going 65mph. I am at a loss. The transmission shop (the best and most reputable in my area) said it is not internal. The truck service company that normally works on the truck says they don't do transmission work. I am so stuck and this is the backbone of my business and just spent $3000 in repairs at beginning of the season for new turbo exhaust ect. Forced me to park it and go back to a truck and a trailer.