93 Caravan Transmisson drops out - electrical

While driving - usually on the highway - a 93 Dodge Caravan just seems to
intermittently slip out of gear into neutral (or maybe 1st gear?) - more
likely to happen on the highway or when driving at an even speed. If you
turn the ignition off and then restart the car it is back to normal, but it
will not correct itself until you restart the car. The tranny was replaced
with a rebuilt unit 30,000 miles ago. It has been back to the transmission
shop (and a good one in the area) and they replaced (for free, believe it or
not) the solenoid pack, and now the PRNDL switch. The thing still does it.
They say it is an electrical problem and are out of ideas. They told us our
next step would be to go to a dealer who may replace the entire electrical
system to repair. Not an attractive option to say the least. Does anyone
have any ideas or things to check?

Thank you in advance!