Should be an A500.
Sounds to me like something broke inside the trans.
A speed sensor would mess up the speedometer, and may cause strange stuff with the motor running, but I doubt it would cause a problem with the trans function other then maybe loosing OD. The system is OBD I, its pretty simple.
I think I would drop the pan and see if maybe the band lever fell out of place, or see if there are piles of stuff in the bottom of the pan. Gene