Helping out a friend with a '65 Imperial, so it's a 727 with a cable-operated shifter. Before I get into the problem let me first say that the trans was rebuilt by somebody who's knowledge/abilities related to things Mopar are questionable.

The problem: transmission does everything fine until you've been driving for over 45 minutes or so and have really warmed things up; at this point you can select whatever gear lever position you like but the transmission behaves as if it's still in drive - no neutral, no reverse. When you shut the car down and wait around 15-30 minutes (it varies), start her up and things are back to normal. Two other observations made while the car was in the "drive only" state: 1) manuallly selecting second or low seem to work ok (no upshifts), 2) ignition/starter circuit was open regardless of gear lever position indicating the neutral safety swith wasn't getting triggered or connection was unreliable (ended up bypassing to remove it from the equation).

I have some ideas but was hoping somebody might have a similar experience to share, then compare notes.