RPM goes to '0' on 91 300SL
 

Interesting dilema! While cruising at a moderate sustained speed, when I take my foot off the gas pedal, sometimes the RPM's drop to zero. If I press the gas pedal, they come up to normal again. Happens very rairly but I'm fearful it is the sign of an emerging problem. Does this sound like a sensor of some sort?

OK, if I add more information maybe I'll get some hints from someone! I think the engine actually stops running when the RPM's drop. But, if gas is applied when it is still moving, it starts again. Could this be a fuel system problem of some kind? If I bring the car to a complete stop, it is not running and needs restarted.