Not a mopar but still need help

These intermittent problems can be as simple as shift linkage worn through a wire.
In many cases a trans service shop has seen the same conditions often enough to know exactly where the problem is.
Here is a good example... trans rebuild in my bros Mustang cost 900.00. If he had went to anyone and had it diagnosed rather than continuing to drive it, replacing a 25 cent bushing in the throttle pressure linkage up at the throttle body could have saved the trans and the 900 bucks. The failed plastic bushing was replaced with one made of bronze so the same failure wont happen again. We live and learn.
Good luck with it.