Tranny Downshift Problem - Please Comment

Car is 1992, 3.8 litre engine with less than 60k original milles. First of all, the PCM will not communicate with a scan tool (am told this is a common issue with older ford taurus cars), so I don't have access to any trouble codes that might be stored in the PCM. The problem is that the transmission will sometimes fall out of gear at low speed. At highway speed, it will also sometimes fall back into low gear. Check engine light is not on. I removed the trans and disassembled. Internals of the trans looked to be in pristine condition with no obvious wear on clutches, bands, drums etc. Fluid was clean and no debris in the pan. I did replace an aluminum piston with steel piston but there was no obvious signs of problems on the aluminum part. I also changed the park/neutral safety switch but this did not solve the downshift problem. Could a defective PCM be the root cause of this transmission problem? Should I replace it? Any idea where I might get a wiring diagram so that I might check to see if a bad wire connection might be the reason the scan tool will not communicate with the PCM? If I can't fix this problem, I'll probably end up junking an otherwise good car. Please Help!
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You may want to check the vehicle speed sensor to make sure it is working properly.

-Rod

You don't need a code reader to axcess engine fault codes on a '92 ford product. With the key off, find the computer test block (firewall, passenger side of engine compartment) and use a clip cord to connect the gray/red wire to the brown wire (brown wire is on a seperate pigtail). Now turn the key to ON (don't crank) and read off the stored error codes from the flashing CEL light on dash.

Stored error codes are flashed in groups of 3. It may take a few tries to get used to reading the faults this way ... LOL.

Not exactly on the techinical side of things, but people swear by this Lucas tranny additive. Might be worth a shot to put some in and see if it helps.
http://www.lucasoil.com/testimonials/index.sd?catid=15