My sons 1997 ranger (2.3, 5 speed, 130K) clutch works fine 95% of the time. However, 5% of the time after putting it in gear and releasing the clutch pedal, the truck doesn't move. Gradually the hydraulic fluid moves back and it releases, sometimes suddenly at the very end, and then it works fine. I also noticed that when this happens there is a buildup of hydraulic pressure at the pedal and it only goes about half way down. I haven’t done anything yet, wanting to get ideas first.

 

Description makes me think the slave is not retracting all the way all the time. If it were partially extended, you would run into a hydraulic lock in the downstroke of the pedal, just as you describe. It can't extend any more, and your pedal pump is trying to expel XXcc of hydraulic fluid on a full pedal movement, and the cylinder already has YYcc of fluid as it is partway through its movement already.
You can try bleeding if it is munged fluid, or the check valve in the master is not allowing the fluid to get pushed back into the reservoir, or the slave cylinder is binding due to corrosion, dirt, etc.
tom

 

I had the same thing happen a few months ago. Changed the slave cylinder and no problems now on my '97.

 

Change the slave.