I know you said that you checked powers and grounds, but how did you do that. With an ohmmeter, or the voltage drop method? An ohmmeter will just tell you whether there is continuity, voltage drop will show you whether or not the conductor can pass current. If you need instructions on voltage drop method you can Google it or we can walk you through it. It really seems like a bad ground or a bad connection at the PCM. If both PCMs work the same, that probably rules out a bad PCM (maybe). I am not sure about the P series vans but the C and K series pickups had a major ground at the thermostat housing that was really bad for corrosion. I usually solder a fresh terminal on the wires and move them to an intake manifold bolt.

The transmission going into "neutral" is something else. The electronic shift transmissions will go into failsafe mode, which I believe is second gear. This would seem like neutral because the engine would rev quite high. That would be an indication of a power or a ground issue. If it was truly going into neutral, if you didn't accidentally manually shift it into neutral, it would most likely be hard parts inside of the transmission. Hard part failure is usually consistent though, not intermittent.

Hope this helps, Jeff