Ah! I know the answer to this!!! I had this nightmare with mine! There is a special double serpentine belt kit with a double grip belt and a special tensioner designed just for this problem. I can't remember what its called but google is your friend. However, don't go this route until you have replaced the tensioner arm. What happens when it loosens or wears out is that it cocks ever so slightly. Not enough to really notice it, but enough for a design flaw plus no water shield (another design flaw) to make every puddle your worse nightmare, (the belt just slips right off with splashed of water or snow).
Let me know here if you need help hunting down the name of the kit and manufacturer, but try replacing the tensioner arm first and make sure the new one is calibrated properly.