Yeah like I said I am dumb when it comes to cars and this was years ago so I may misremember. All I know is that if I got my car up over around 50mph it would shake violently and stall when I stopped. Then if it sat for 10-20 minutes it would be ok again (until it got in 4th again). I do know they disabled something that reduced my mpg slightly and I never had the issue again for the 2 years I still had the car. Again I don’t remember 100% but I think he literally just unplugged a cable. Charged me 75 for diagnostics and 25 for labor.
Actually reading your comment again “Torque Converter” sounds really familiar so that’s probably it. If we are running a pool I got $5 on stuck torque converter lockup clutch.