The base idle has to be reset EVERY TIME the throttle body is cleaned, so the throttle position sensor can properly modulate the idle control motor, but I wouldn't be suprised if the dealer didn'y know that. You can easily check for vacuum leaks with a can of gum cutter/carburator cleaner by spraying the intake manifold at the cylinder head while the car is running, if the idle improves, the intake gasket (which is a common problem on these cars)is leaking, it is most common that the rear cylinder is the culprit. Also if the distributor is in the back of the head, remove the cap and check for oil contamination from the distributor seal inside the unit leaking. The engine "rpm" sensor as you call it is only used at start up to determine crankshaft position, so if the car starts it is working, hope all this helps. I work on these cars for a living and they're not hard to fix 95% of the time, it's just finding someone that knows what the hell their doing thats difficult, good luck with the dealer, you'll need it. SEE-YA! --LUBEHEAD P.S. not all dealers are bad or the mechanics that work there.....just most of them.