One thing it could be is the blocker plate is worn. Mine did that, with the exact same drivability problem. The only whay to tell for sure is to take it apart, though. Could be somewhat difficult to get a new blocker, they're not being made. The manual says you can squeeze it together if the tension is weak to restore operation, mine was beond that. Got lucky enogh to find another trans to rob parts from (the main box was trash, od ok). I had a few other issues, to. A peice of the blocker broke of and damaged the housing, but that was secondary t the actual problem.
Check out all of the electrical and external controls first. If they check out, most likely the blocker is the culprit.
