Is it possible that the control cable is vibrating backwards over that time, closing the valve in the vacuum motor?
So when they stop can you restart them by turning the knob on the dash?
Alternately it could be the seal inside the motor itself. The paddle that is driven by engine vacuum could be worn and get worse as it heats up through use.
This guy
http://www.wiperman.com rebuilds them.
Ficken Wiper Service
132 Calvert Ave
West Babylon, NY 11704
(631)587-3332 M-F 9-5 EST
Robert Ficken
sales@wiperman.com