chatter usually happens when its not installed correctly. that happened to my scion tc i had. i had a spec stage two that would chatter almost all the time on the highway. sounds almost like one of the springs in the clutch where slamming into the flywheel. In more recent experiences i had a stage 3 spec in my legacy gt ltd, that chattered ever time i started in first gear. deffinately to strong for the car, it held back the engine quite a bit. Maybe look at getting a different stage clutch, or getting it checked by a different mechanic.