I can save you some trouble I think. My 93 850 had the random abs activation. Stops, starts etc. After replacing pressure valves, all four calipers, and the ABS control, I found out it was the abs signal ring on the passenger front wheel. It appears that over time the rust builds up under the ring and apples enough pressure to crack the ring. The ring will be held in place, but with the crack, the pulse codes will be off. Over time the ring will losen even more and the crack will get bigger throwing the fault to the ABS sensor.

Before you get knee deep, look at the front two ABS pulse rings, Check them over carfully, it is sometimes hard to see the crack. If you find the rings are cracked, you will likely need to replace the shafts with new or rebuilt units. I replaced mine and solved the problem. I wish I would have found that first and saved all the other expense.