Vehicle speed signal in Typ 89

[radek at istar.ca]

Hello fellow Audifans.

This weird thing has been happening to my 88 90Q lately:

from time to time, while driving, the speedometer needle starts to oscillate (about 5 km/h
amplitude)
and the fuel consumption display goes crazy, showing consumption three times higher than
normal.
There is no noticeable difference in how the car behaves.  The problem comes and goes
without
any pattern.  Overall fuel consumption (mileage between tanks) seems normal.  No error
codes.
Is the vehicle speed signal screwed up somehow?  How can I test it?

I should add that the O2 sensor is nearly new, the temp. sensors (both, the upper and the
lower) have been
replaced, but the car idles high (about 1000 - 1100 rpm.).

I'll be grateful for any tips, this thing drives me crazy.

Radek
88 90Q
91 V8Q