As I understand it, you say the motor runs fine out of the truck, but when installed it slows to a stop.
I agree with Cletis's recommendation to reduce the friction between the blades and the glass. Don't forget to reduce the friction within the linkage, too. As I recall, mine is a rather low tech assembly having only bushings, no bearings, at the joints. Make sure you've reduced friction between the motor and the blades by lubricating those joints.
The only thing left is the vacuum level and your motor. Is it possible to have vacuum restricted through the knob? You have good vacuum on the engine side but maybe crud can build up in that little valve and starve the motor.
Just a thought.
Bill
