I've got an '83 pickup that I've replaced the in three months ago. I noticed two days ago a strong vibration in the seat when going over 65 mph. If I press in the clutch the vibration goes away. If I release the clutch whether in gear or neutral the vibration returns but goes away under 65 mph. My fix is to not go over 65 mph but I'd like to know what I could have done wrong with the clutch install. It works flawlessly. All I can figure is something on the pressure plate.

Thoughts?