Sounds like a bad mixture rather than tranny problems. Putting into reverse will require a transient load from the engine, suspect the starter motor will have enough torque for the transient when you start the car in reverse. A bad mixture (or timing maybe also?) will cause the idle to drop that far when you select a gear. Can't remember, is it electronic or mechanical FI? My B20B with Strombergs and BW35 drop about 100 RPM or so when selecting a gear (measured by ear, no rev counter on my car).