Interesting! I am going through the same or similar problem... My 2003 Forester has been idiling erratically and high. At minimum, it would idle roughly, the needle bouncing around 1200-1500 rpm. With more driving, the engine would, with no pressure on the accelerator pedal, repeatedly rev up to 3000 to around 4000 rpm, sometimes even staying at those high rpms for a while. I've never had the car stall, though. When I would unhook the battery for 20 minutes and then hook it back up and turn the car on, it seemed to idle fine for a while, but after driving for a bit, I got erratic and high idling again. Still no stalling, though. I checked the codes and got P15-14 (Throttle Body Performance), P0113 (Intake Air Temp Sensor High Input), P0032 (O2 sensor heater circuit high), and P0519 (Idle Air Control System Malfunction).
Today, I cleaned the IACV and reinstalled it. No difference. I still need to check the codes again after reinstalling and disconnecting the battery. I did notice, though, that the gasket has some damage. Perhaps it is a vacuum leak. I will try your method later today or tomorrow and let you all know of my results.
