Transmisson Problems

[jdavis763]

I have a 1992 Sable, 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 and mercury sable 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 when going downhill with foot off the accelerator. 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. Did replace an aluminum piston with steel piston but there was no obvious signs of problems on the aluminum part. Could a defective PCM be the root cause of this transmission problem? Should I replace it? If I can't fix this problem, I'll probably end up junking an otherwise good car.