1998 BMW 740 Windshield wipers

The windshield wipers on my 1998 BMW 740il stopped working months ago, and then suddenly started working again, but only when the car is in motion. For example, sitting in the driveway after starting the car the wipers do not work. Driving down the road thereafter they come on, somewhat sluggishly, but basically function normally. They seem to continue to work somewhat normally, but when I come to a stop sign or red light, they stop working, usually in midwipe. After I start moving again they come back on. This has been happening consistently for a few months. Most mechanics I talk to say they have never heard of this problem. My local dealership was ready to replace the wiper motor (for $850), but the motor is broken why do the wipers work normally when the car is moving?