What could cause a new clutch to work at first and then disengage at freeway speed on a 84' ford ranger 4x4?

It had been working great for a few days, When it was in gear and having the clutch pedal up, it slowly over 2 miles stopped working, it then was acting as though the clutch pedal was in. I then put a new master and slave cyl. [as it is hydrolic], and after bleeding it there was still no change. I checked the torque on the pressure plate and it is good. Also I noted that it made no noise when all of this occured but a slight scent of clutch burning as it initially decelerated.