Transmission problem
1995 Ford Aerostar 6 cyl Two Wheel Drive Automatic 132000 miles
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My parents bought a used Ford Aerostar. 95 V6 3.0L.
The van sat for about 18 months, but drives fine, no apparent issues, good mileage, etc., and good acceleration and upshifting.
The problem is that when stopping suddenly (fast enough to make items slide off seats, threshhold braking but never hard enough to lock the tires) the transmission does not shift back into first gear. In fact, the transmission stays in a high enough gear that the engine will chug and the car will die.
Putting the van into neutral and letting it coast, or simply backing off the brakes enough to let it coast the last 5 or 10 feet will prevent the problem, but this is not practical.
This ONLY happens when the car has shifted into 3rd or 4th gear. It will not happen under 30mph.
Normally the van will restart fine, but sometimes it has to sit for a minute.
I changed the Tx fluid (mercon ...4? it was expensive) by running the car and using the transmission coolant lines (the problem existed for a long time before this) like I have done before on my volvo without problems:).
The tranny is a 3spd+OD and I was thinking several things:
1) The trans is shot. I doubt it.
2) The transmission is electronically controlled and needs to be reset (tried leaving the battery cable off for days, no change)
3) The transmission control valves are worn or need to be serviced. (assuming 300-500 $ job)
4) The filter is plugged and causing it to not downshift
5) The overdrive is not disengaging. Disabling it will make it useable around town, with high fuel consumption.
6) The car (?)has(?) a locking torque converter that is not unlocking.

One thing to note is that the car has smaller wheels and tires than intended (1 inch off). Any ideas? I didn't want to drop the Tx pan, but I doubt a filter is to blame.
My parents are trying to sell the van and it would definitely increase the value to decent if we could repair this issue.
Thanks for any help,
Will