1989 Chevy Truck run away idle

Upon startup, still in park the idle rpm raises about 1000 rpm above normal, after a couple of minutes the idle settles down to normal. Always figured it had something to do with whatever is acting like a choke, (throttle body injection)messing up, or maybe this is normal. Cold or hot weather it has the same problem.I can drive around all day, get the engine temp. To normal range and stop at a light and the engine will try to runaway, have to hold the brakes harder. Than it will calm down after a minute or so. When I go to park the truck the idle runs away again for a couple minutes before it settles down.I normally dont allow it to run after I park it because of the excess rpm and just shut it down. Any help is appreciated.