91 Caprice running rough

We have a 91 Caprice (89,000) that is running rough. If left in park the engine seems to run steady with no problems. As soon as I put it in drive the engine starts to run rough and pulling away it still runs rough. Driving down the road it seems good but when stopping at light it will run rough again. Things I have tried are new plugs,cap,rotor,air filter,fuel filter. I have cleaned fuel system (regane),throttle body,egr,idle air control.
Any help would be greatly appreciated as this car is now slated for the bone yard and I would hate to see it go.

there is a gizmo that bumps the idle speed up when you turn on the a/c or put the car into gear to keep the thing from stalling out due to the increased load. i cant remember its name, but i would start there. it is a solenoid connected to the throttle in the engine. it extends to bump up the idle speed by pressing the throttle open a tiny bit. describe it to someone and they will kno what im talking about.

Throttle position sensor?

Throttle position sensor?

No - I forget what it's called, but it will have several wires going into it and it should be on the driver's side of the TBI unit. There's also the IAC solenoid that you can check (idle air control).

its called either the throttle positioner, or the ISC (idle speed control), which ever you have.