What's the condition of your alternator? Sometimes under a full load such as lights, heater at full blast, heated seats, radio all on, the wipers become slow / sluggish. I've never seen it stop / start across the windshield under full load; just sluggish and slow. So perhaps your problem may be in the mechanism as the other posters have suggested.

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ARVY

1999 E430
1999 C230 Kompressor
1992 300E (write-off)
1984 190D 2.2 (sold)