I would try to get a code reader or INPA that shows individual wheel speed sensor operation. I know you said you "tested" it by hitting brakes in snow, but they could still be faulty. I was having an issue, checked speed sensors with code reader and they all showed good, but I decided to change them anyway due to age and the problem went away. The issue is that the whole system relies on so many parts working in harmony and if one goes away, you lose everything. When mine wasn't working last year I took it out in the snow and it was the scariest car I have ever driven in the snow. Would just flip ends, skid without notice. I fixed that wheel sensor and it is pretty good now, not the best, but better. I would also recommend getting 4 snow tires for winter time. Made a huge difference on mine. Good luck. Can be very frustrating

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600