A similar problem has plagued my 93 960. My wife's 97 960 has no such problem and has a super smooth idle. Maybe it's the later version of Bosch Motronic fuel injection but I doubt it.

So far I have changed the intake manifold gasket, fuel regulator, throttle position switch and fuel pump relay. There have been small improvements in idle but the roughness is still there.
The intake hose was eaten away at the flame trap housing. I cut off the remaining flange and jammed the housing into the hose elbow, there's enough rubber to make a seal. Also used nonhardening sealer and a piece of wire to bind the housing to the elbow-works a treat-no more oil leaks and smoother running. New intake hose is $150!
I also discovered the airbox thermostat had failed, resulting in heated air only. I have jammed the air flap to the ambient air position (with a terry cloth) and can imagine it helps idle smoothness and pickup.
Tomorrow, I plan to change the rpm sensor, what the hell I have nothing to lose!
I will also replace the present Bosch +4 with Champion copper core plugs as the latter are closer to OEM plugs. They may also cure a low level whine present on the radio (I have already replaced "engine" and injector relays located on the strut tower.

Good luck and tell me if you find the cause of this small but annoying problem,

Dan