1990 Grand Marquis GS 5.0 EFI

Starting this summer, every once in a while, after driving on the highway
the idle would go way up. It would stay high until I shut the engine off.
When I shut it off the idle always goes back to normal.

This high idle doesn't happen every time I get off the highway but enough
times for it not to be normal.

Any help in directing me to what is sticking to make the idle run high
under these driving conditions is greatly appreciated.

Besides this problem the car runs great.

Thank you.