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"ABS off" warning light problem

I've been experiencing the "unsolicited ABS unlocked warning light
illumination" syndrome on my '86 5KCSTQ.

I recall some previous posts about this, but I'm a little confused about 
what causes this lamp to light.

Apart from the desired causes of manually disabling ABS (dash switch) and 
locking the diffs, what caused this?

This problem seems to be exacerbated by cold and/or damp weather.  I saw it
a couple of times last winter, it was fine all summer, now it's started again.
Up until recently, it manifested itself only at start up and was cured by 
toggling the dash switch a few times.  Now, in addition, it seems to come on 
randomly while moving.  It can still be extinguished by toggling the dash 

I assume this is the fail safe mode when the control system defects an error
of some sort.

Previous posts suggest checking the adjustment of the wheel speed sensors.
I can see why this could cause the random problem, but not the start up case.
If the wheels are not rotating, there is no output for the control system 
to check.  I was speculating (hoping!!) that the whole thing could be explained
away by yet another flaky Audi relay. As there are at least three (to my scant
knowledge) in this system, I defer to the list experience base to point me in
the right direction.