Engine stumbles when put into park

MY 87 4.0L has a new trick. When I put it into park it stumbles and just about dies before catching itself and returning to normal idle. It only does this when put into park. Any thoughts on what would be causing this?

Clutch not disengaging all the way? No clue really..

Ooops...Forgot to mention, this is with an auto tranny

See if it does the same thing when you do the very same routine but put it into N instead of P. Either action will abruptly drop the torque converter load and cause the engine to accelerate briefly. Wild first guess would be a little vacuum leak or throttle-position sensor issue that is causing it to stumble when it unloads. Try also abruptly tapping the accelerator when it's idling in neutral or park, and see if it stumbles then.