95 A90Q Anti Lock switches off Backing up intermittently

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ABS appears to int lock up RR wheel backing up and Anti Lock Off lamp comes 
on, especially if the wives car has been sitting for a week or two.  Typically 
unlocks after driving forward a 1/2 mile or so or alternating backing up/going 
forward.  

Years ago I repaired another ABS oddity on an A4Q by changing a rear outer CV 
joint that had a cracked reluctor wheel but the LR outer joint is recent on 
wives car and I removed the RR and it is clean.

Has anybody experienced/repaired this?
Thanks