[12Ounce]

I suspect that a ball-spring-detent positioner internal to the tranny needs some repair. Probably not a big job, as tranny repairs go. And my comments here are not much help I know ....

... but I would suggest that the tranny (in Park) is not to be used as a hill-holder. That's what the manual brake is for. The tranny is just to be used as a fail-safe back-up for the manual brake. This can be verified by reading the owner's manual.