71 Dart 727 w/suregrip having weird issue

Yes it was reverse as well. I think it was overheating, after letting the car sit in the garage for 2 hours I started it and moved the gears and it seemed normal. The old AC condenser was there so I removed it to maybe allow air to move more freely. I also may add a trans cooler or at least an electric fan on a switch. I never had this problem with any of my other cars......hot engines maybe but never hot transmissions so I was taken by surprise a bit there. If it ever stops raining I will take it on a loop around the highway and see if what I did helped any.