Phil wrote :
"When a car sits a lot the accumulation of dust absorbs humidity. That stuff gets where it wants to go.
Then, It can cause some rusting action to what should otherwise be smooth machined surfaces.
This also affects the input shafts splines and the internal splines of the fiber disc, that is supposedly meant to float in between the flywheel and the pressure plate surfaces. The splines get choked up by sitting outside in an open environment quicker too."

I beleive the problem with your clutch is right there with also possible clutch cable adjustment. When putting in a new clutch disc/pressure plate (and throw out / center bearings while there) the problem will be gone for a very long time.

Tell us what you found.