93 Corvette Clutch issue

Please excuse the length of this topic. I am totally stumped and have
tried just about everything I can think of to solve my problem (hence
the length). I have a 93 Corvette with the ZF-6 manual transmission.
The car is totally stock with 50K miles. I've owned it for 18 months.
Right after I purchased the car I noticed the clutch pedal was
difficult to depress and it didn't shift well. Two shops said I needed
a new pressure plate. Last summer I changed the pressure plate, disk,
throwout bearing and pilot bushing. It approved the shifting. However
as I drive the car the clutch pedal stiffens up and the car does not
shift as well. I've changed every possible part I can imagine. Here
is a list:

Clutch master (twice)
Clutch slave (three times - did run into a manufactures defeat on two
of the units)
Braided clutch line
Clutch pedal pivot bushings
Throwout fork
Pivot ball
Pressure plate (twice)
Clutch disk (twice)
Throwout bearing (twice)
Pilot bearing

The throwout bearing guide tube is in great shape with no ridge or
scuffing. I've literally been battling this problem for 18 months. I
am totally stumped. Has anyone ever come across this scenario or does
anyone have any advice?

Thanks in advance for your help.

Jim