86 Audi ABS will not stay engaged

[John Nompleggi <jnompleg@cybertours.com>]

I have a 1986 audi 5000 Quattro, standard transmission.  The ABS (anti lock
braking system) will disengage itself after driven approximately 1/4 mile.
We have checked the hydraulic system presure switch, adjusted/calibrated the
speed sensors and replaced the computer module.  The regular brake system
works fine and both the center and rear axle differentials operate normally.
When  the differentials are engaged the "ABS off" light comes on, indicating
the system is off, as it should be and when disengage the "ABS off" light
goes off as it should but only for a short time and then comes back on.  At
speeds above 50 MPH if you manually engage the anti-lock braking system, by
pushing the ABS switch twice the light will go off indicating the ABS is
operative.  The System does engage) and will remain operative until the
speed drops below 50 MPH.  Has anyone had a similar problem and if so how
did you fix it?  Any help will be greatfully appreciated.

                                John