Ford - Ranger :: 1995 - Clutch Does Not Engage / Pedal Starts To Feel A Little Spongy

I have a 1995 Ford Ranger 4 cyl w/ hydraulic clutch, about 95k miles, and an intermittent problem. Normally, my clutch works fine, but a few times a year, normally during the summer, and normally only after driving for a half hour or more, it starts to act up. First, the pedal starts to feel a little spongy. Then, I'll depress the clutch, and when I release it, the peddle comes up, but the clutch does not engage; a few seconds later, the clutch engages fully instantaneously, which usually stalls the engine if I am stopped.

Then, when I try to reengage the clutch so that I can restart the engine, the clutch is very stiff, and I have to pump it a few times, at which time it will depress easily, and then I am usually back to the original problem where the peddle comes up but the clutch does not engage. When I finally make it home and the truck sits for awhile, the clutch then works fine again. I think it is the orig clutch. I have talked to several mechanics about this problem (including the Ford garage!), and they are mystified. One mechanic thinks that it is a problem with the master and slave cylinder combo, and can replace them.