Have your tried different methods to avoid chatter?
More or less throttle, and faster, or possibly slower on letting the clutch out?
A friend of mine had bad luck with new clutches chattering in a car he was working on, everything was done right, ground the flywheel too. Eventually, after several different clutches in a week or so with no fix he just drove it for a few days, trying to minimize the chatter when taking off, and in no time at all it was cured, just needed driving. Doubt that's your problem, but something to think about.
Grigg
