Automatic Transmission problems of the 1997 BMW 528

On several occasions while driving during wet conditions, the automatic transmission would suddenly shift from "drive" to "s" without any operator intervention. When this occurred the car abruptly slowed to about 25 mph and would not accelerate beyond that speed. I wish to emphasize that this sudden and unexpected change in gear selection would take place without any operator intervention which resulted in a potentially dangerous situation. I had the vehicle repaired at vista BMW in coconut creek, FL on 9/07/05 and was told there was a short in the gear selector switch and the defective part was replaced at a cost of $583. 25.