96 Ranger 4 cyl, original owner 25K miles.

I have an internittent problem with the clutch.  It works fine most of
the time, but  now and then the clutch pedal doesn't want to depress.
Seems to happen when truck is driven more than 30-40 miles.

When this happens the pedal goes about half way in and stops without
disengaging the clutch.  It feels like it is hitting something solid
and feels about the same as pressing on the brake.  When this happens,
I can free it by repeatedly stomping on it until it depresses all the
way and releases.  Usually takes a minute or two.  Then the pedal
comes all the way back up, but the clutch doesn't engage or else slips
a lot like it is barely engaged.

Once or twice, it suddenly released while the pedal was all the way
out and needless to say caused quite a lurch.

I think it is the release mechanism (we used to call it the throwout
bearing) sticking.  As you can see from the mileage after 10 years, it
doesn't get a lot of use and quite often may sit for a week or so
without being run.

This is a hydraulic clutch release and as far as I've been able to
determine, it's located inside the housing, not outside like slave
cylinders used to be.  The fluid is full.  

Any advice on cause of this problem?  If it is a sticking release
cylinder, is there any way to lube it or free it up?  If it needs
replaced, I will probably do it and chage the clutch plate at the same
time.  Any tips on doing this on this truck?  I have changed many
clutches in the past but all were pre 1970 models of various cars.

Thanks for any help.