I can tell you what happened to me. I had a four speed in my dart and similar instance where the car would run then seize sitting at Idle. Ended up being the input shaft bearing in the front of the trans. I think it is possible that something is seized in your trans. The only way to be sure is to heat it up until it seizes then remove your converter bolts and see if you can turn the engine with a breaker bar. remove your inspection cover while it is cold as it will be hot after you get it to seize. It sounds like work but you don't want to tear into your transmission unless you are positive that's where the problem is. Although if it is seized you wont be able to get the converter bolts out DOH!!!. Disregard...