Thanks to Dan, Peter O., Larry and everyone else who replied to my posts...
'89 Jetta, when cold, idle oscillations between 800-2k RPM.
The VW tech. found two problems: the hall sender is loose (apparently it had been glued in the past) and the knock sensor wire is cracked and needs to be repaired.
The knock sensor wiring has been bad for as long as I've owned the car and the dealer has seen it in the same condition each time I've had the exhaust gases checked (probably 3 times during my ownership). Is the sensor easily replaced? Looks like a 13mm bolt. Simply unbolt and torque (to spec.) a new one? I'll try repairing the wiring first. Maybe use some of that plastic ribbed shielding? Heat shrinkable electrical tape? Suggestions?
From the name, I'll assume that the Hall sender *sends* ignition signals to the computer? This sounds like the likely culprit. When the connector is jiggled, the problem apparently resolved itself--although operating temp. also 'fixes' it. I'll pick up a new distributor tomorrow yet not at dealer prices ($310 plus $114 for installation).
I'm still all ears, Darryl.