I had a vehicle that did something similar to this once and it was a cylinder that was bad. It cost like $400 to fix but got me a lot more life out of the vehicle so it was worth it!
I had a 1991 that would not up shift and it was under warranty by 200 miles. It needed to be rebuilt. These transmissions are one of the weak spots on the van.
Are you sure the fluid isn't getting too hot and thin. I would get a trans. fliud cooler and mount it in front of the fan. They aren't expensive or hard to put in.
Are you sure the fluid isn't getting too hot and thin. I would get a trans. fliud cooler and mount it in front of the fan. They aren't expensive or hard to put in.