Strange Speedometer Behavior

["Robert (Bob) Myers" <RMYERS@WVNVAXA.WVNET.EDU>]

A little Net-Wisdom, please?
This morning my '89 200TQ's speedometer started behaving strangely.  The
needle sort of "bounced" between two readings like, for instance, 25 and 35
when I was driving at an estimated 40+/-mph according to the tach.  The period
of the bounce was about 1 second.  Increasing the speed decreased the magnitude
of the bounce and also decreased the period.  (The bounces were smaller and 
more frequent.)  During this period, which lasted for about 25 miles, the 
cruise control would not operate.   I finally had to stop at a light and when I started up again the bounce was gone and the cruise control worked.  

Turning off the ignition for a short time while rolling at highway speed did
not stop the bounce.  On restart the bounce was still there.  (Yes, I know, the steering wheel is locked with the ignition all the way off.  This was only on
a straight stretch of road.  :-)  )

This behavior has happened a few times before but does not happen on every trip.
Other than the "bounce" of the speedometer and the related cruise control
behavior, nothing else out of the ordinary was apparent.  The voltage needle 
was rock steady.  Oil pressure varied some with engine speed but that has been
consistent since I've owned the car.  Nothing else seems amiss.

Any suggestions?  TIA.

Bob  __________________________________________________      ^_____^
    | Robert L. (Bob) Myers  <RMyers@WVNVMS.WVNET.EDU> |    /       \
    |__________________________________________________|   (  O _ O  )
            Love M'Audi - Love M'Bimmer.                    \  (_)  /
       Siberian Huskies are better than either!              |  U  |