pb shifter won't go into reverse

On my "new to me" 64 Dart , the push button auto shifter won't go into reverse, seemably only when the engine and trans are up to temp. In other words, when I pull out of my driveway, it will go into reverse, but when I bring it back after 10-15 mins of riding, I can't get it to go in reverse. Also, my engine is reading rather hot, so I will swap out thermostats first. I have a 190 degree on there now. Thanks