I submitted this question a few days ago and don’t know what happened to it. I recently acquired a 1990 Coachmen class B motorhome, built on a Dodge 1 ton chassis with a 318 v-8 and a four-speed overdrive automatic transmission. After about 30 to 60 minutes of road driving, it starts to shift into third gear unprovoked. That is, for no apparent reason, uphill, downhill or flats, at highway speed it just drops into third, then vacillates back and forth from third to fourth for a few minutes. Then it settles down and drives normally until the next episode, which could be in a few minutes or a few days. Also, I have noticed that I can’t get it to downshift by stepping hard on the gas pedal unless we are going quite slow.
I had a co-worker that had the exact same problem with a Dodge, Intrepid. After a long period of time taking it back to the dealer, it was found to be the ignition coil. The secondary windings were arching sending the wrong information to the computer. Hope this helps, good luck.
Thank you. I will have that checked out.