Have you checked the earth on the road speed sensor? I had a similar issue on my 94 +8, it started by on occasion stalling when coming to a halt and progressively got worse over time. On locating the road speed sensor I found it had become loose from the bulkhead and not giving a good earth, cleaned up the bolt tightened and never had the problem since. If the road speed sensor is not functioning correctly when the car is coming to a stop the ECU does not react quickly enough to the slowing of the vehicle which the sensor would tell the ECU and hence it can stall.
