Sounds like it could be 1 problem causing another. Perhaps the shift cable was adjusted too tight for quite some time, causing one to have to muscle it into the park detent on the shift lever, which overtime may have caused it's detent to wear to the point of allowing it to pop out of Park. I think first I'd try to get the shift cable adjusted properly and see how it reacts there. Then if you still have issues disconnect the transmission shift linkage from the transmission and make sure park is working properly on the transmission, then on to the shift lever.