My friend's 940 has a very consistent stumble between 1500 and 2000, whether you are accelerating or holding throttle, accelerating hard or easy, or in gear or not. It runs smoothly below or above that threshold. Just replaced wires and checked plugs. Ditsributor cap and rotor were terribly pitted and worn. Replaced them. Smoother idle and more power than before but no change with stumble.
Might this be a sensor of some sort?
Glenn