"Classic" Problems

My 1990 RR has some strange problems and I need help in identifying them. When going downhill in a low gear with my foot completely off the throttle, she surges with rpms climbing up and then down. Usually, the rpms start out low and normal then go up after a few seconds so the engine-braking effect is completely negated. I have also noticed this surging when in cruise control at speeds of 30-45 mph. Vehicle has a relatively new long block, fresh oxygen sensors and plugs, plug wires, etc. I suspect a faulty road speed transducer but that is just a guess since nearly everything else has been replaced. The vehicle has been driven on the beach for years and, although her undercarriage gets hosed off after each time, chassis rust is present and I think this transducer could be suffering rustitis. I also notice the fuel consumption seems to vary widely from an average of 15-16 mpg to 10-12 mpg. I get especially poor mileage at low speed. Any ideas??

Clean out your 'idle control sensor' carefully and thoroughly. Use a q-tip, and don't force anything. Good luck.

Thanks for the suggestion but do you mean the idle control valve (solenoid)? I have never heard of an idle control sensor.