Mystery Transmission Problem - '98 Windstar

Rebuilt the transmission on my Windstar about two and a half months ago. Exactly two months after the work was done started having transmission trouble again and the CEL came on.

Here's the list of symptoms:

-Slow to shift when it's cool.
-Once it's warm (20 min, roughly) Spontaneously downshifts, revs up to 4500 -5000 rpm, for no reason at all.
-Comes and goes after that, doesn't matter what speed I'm at, uphill, downhill. In fact, I tried coasting down a steep hill (still in gear). I started at 45 mph and it slowed down as I went down the hill.

Took it back to the transmission shop and was told that there were no transmission codes, only engine codes. Maybe it has something to do with the engine. Get the engine work done first and then maybe they could see something.

Had Ford do the work (sorry, I'm good with little stuff but I'm not touching the engine). After they were done they said the engine is running great but they did notice that there is a tansmission issue during the test drive after they were done with their work.

I drove the van straight from Ford bck to the transmission shop and dropped it back off over the weekend. Still experiencing the same symtoms - shifting, revving. Doing the engine work changed nothing with any of the symptoms.

Only this time, there are no codes! Something is definitely wrong but they can't find it because the D*** thing won't mess up when they drive it and there are no codes!

Does anyone have any idea what could be causing this?

UPDATE: Transmission shop replaced the speed sensor (at no charge, bless them). So far have not experienced any of the same problems.

(this was added to the original post initially. The work was done about a week and a half before this message was posted. Having so many problems I edited it out of the original and gave it it's own place. Like chapters in a book, so progresses my tale of woe.)

UPDATE 2: Had to leave it at a gas station 20 minutes away from home last night. Lost everything but first and reverse. Transmission shop having it towed back to them today.

Still nothing tripping the CEL.

Search the neutral safety switch. I thought I was having transimission failure and it turned out to be a $39.00 switch that goes on the steering column. Found the answer here, but that was a few years ago. So much better than putting in a new transmission and then it still not working correctly!

UPDATE 2: Had to leave it at a gas station 20 minutes away from home last night. Lost everything but first and reverse.

When you put it in D the van doesn't move or move but don't shift in 2nd?

The neutral safety switch is under the hood (behind and under MAF sensor), not in the steering column on Windstar.

I had the neutral safety switch go bad about a year ago. It will drive the transmission nuts. I would be going around 60 mph on a level road and it would all of a sudden downshift into 2nd gear. I replaced the switch for around 30.00 from Autozone or Advance auto can't remember which place ( I use both places, whoever is the cheapest ). It fixed the problem. There is a video on YouTube on how to replace the neutral safety switch correctly. Sorry I don't have the link handy right now.

I read the post again, I'm sure this problem is NOT related to neutral safety switch, since the transmission disengage from 2 to 1, and not from 4 (or 3) to 2 like Riordan3.

It's either a problem with the 2nd gear clutch pack (most probable), or a problem with 1-2 shift valve's solenoid. Of course, it still can be something with electronic like bad PCM or bad contact in plug (but extremely rare for this situation).
But not a problem with other trans sensor.