I have a 94 E320 Sedan with 114,000 miles on it and am having some issues with idle speed:

The car was idling in drive/reverse (without depressing the pedal) at around 500-750rpm, it stalled several times in reverse. The check engine light came on and the OBD gave me Error Code 6 (I had reset it several times and it kept coming back after 48 hours). The car was also noticeably sluggish.

I dropped it off at the shop and they cleaned the throttle plates and the car seemed to idle normally (high if anything) and the check engine light was off. Within 72 hours the check engine light was back on. Idle speed is noticeably higher 1100rpm or so without the pedal depressed in drive and the car lurches forward and goes at a good speed, without the pedal being pressed, more so than when I purchased the car.

Now, most recently, I left my car in park for a short time and heard the engine revving, when I got in the car it kept spiking to 1500rpm from 1000. Now in park/neutral the engine idles at 1500 or 1000 (or alternates between the two)...

Where is the idle speed control sensor and unit, can I check and clean it?... Given the symptoms can you suggest anything else? I'm trying to avoid taking it back to the shop, they didn't charge me this time around, but I'm worried about replacing the idle speed control unit because it appears to be expensive.

Thanks, Jeff