Check for a contaminated or warped flywheel.

Repair or replace the flywheel.

Check for a weak diaphragm spring.

Replace the pressure plate.

Check for an oily contamination of clutch disc.

Replace the clutch disc.

Check for worn splines on the input shaft.

Replace the splines on the input shaft.

Check for a warped pressure plate or flywheel.

Replace the pressure plate or flywheel.