Here is the Clutch Trouble Diagnosis to solve your problem.
Clutch Trouble Diagnosis
The following information is provided to assist you in recognition of the problem(s) based on your clutch performance symptoms, correct diagnostics eliminates solving problems that you may or may not have:
Clutch Vibration:
1. Flywheel out of balance
Clutch plate out of balance
Cover assembly out of balance
Using rigid or rigid cushion clutch plate instead or flexible (rubber, leaf or coil spring) center plate
Bent main drive gear
Clutch plate does not engage flywheel evenly
Backlash from loose or worn universal joints
Propeller shift whip
Misalignment between engine, clutch and transmission
Hope this will help you in solving your problem.
Link: http://www.drivetrain.com/cldiavib.html