If this were a later car, I would immediately suspect the clutch cable. However, for a hydraulic clutch, the problems are either the clutch master cylinder, the clutch fork, or the input shaft splines - inspect the fork carefully for signs of cracks at the point where the pins for the throwout bearing are - it could be flexing there. Also, inspect the tranny input shaft for any signs of notching where the clutch disc rides - these may not be noticeable except under close inspection after cleaning - if you can feel it with your fingernail, then it can interfere with clutch release. Only real solution is to replace the input shaft.