Had suncoast converter and valve body installed about 2 years ago. Been having problems with the truck fully downshifting after pulling a load. A week ago after pulling a few tons of fertilizer I stopped at an intersection, then went to turn... truck was stuck in way to high a gear. Didn't wanna move, lotsa smoke. This has happened before, but this seems to be the most severe case to date. Seems like in the past, after stopping the truck for a while, then starting her back up the problem would go away. Just seems to be getting really bad. I have not changed the tranny fluid since the upgrades, but I tow a few tons only a couple of times a year.
Now with no load..... no problems.

What's goin on??? Any ideas on a fix? Thanks to all. You guys have always been a great help.

 

Sounds like a shift sensor on the tranny to me.