1999 Ford Escort Car starts but can't be pulled out of park

My daughter has a 1999 Ford Escort that really has been pretty trouble free since we bought it used, six years ago.

She just took it to another city where she will only use it sporadically but it needs to be dependable.

Apparently, I am not there, the car worked fine until this past weekend when she started it to go downtown and could not get the car out of park.
She called me and I told her to rock it (thinking in error ignition lock) then to restart the car, and that seemed to work.

It happened again yesterday and when she restarted the car it was fine.

What could it be - somethinig with a vacuum?