2000 Caravan front wiper problem

My 2000 grand Caravan front wipers work in the intermittent mode most of the time but on ocassion, especially when it's raining (duh), they stop working. Adjustment to decrease the delay time has no effect and the only way they can be restarted is by turning off and back on. This has been gong on for a few years and is really anoying. Any ieas?