The solenoid is bad. It is not a problem outside the trans. I have seen this on my 3t40 trans of my APV as well as on both my 80's Buick Regals and when I had a 92 Beretta. The inside of the solenoid collapses over time onto the shaft causing it to stick. Once the trans get warm, the solenoid engages but doesn't release. If you are handy you can dis-assemble the solenoid and drill it out a bit. Or just replace it. If you replace it the problem and come back after a time. Leaving it unplugged affects gas mileage to the downside. You can take the side cover off to get to it without having to take frame members off. The hardest part is the wiring. I just cut the wires and soldered them back together. I'm sure crimping the wires would be okay since the wire splices are not subject to moisture.