Clutch chatter with new clutch

Chatter sucks. The usual culprits are contaminated or heat-damaged friction surfaces. Often, pressure plates and flywheels come from the manufacturer with a coating intended to prevent corrosion. This has to be cleaned before installation or it can contaminate the lining of the clutch disc. As well, greasy hands need to be kept away from the surfaces for the same reason. Chatter can also be caused by the use of puck-type clutches or those lacking hub- and/or marcel springs in the disc.

Assuming your dealer knows what they're doing and used OE parts and took care of the cleaning stuff, you may have other issues. Perhaps there's a misalignment between the engine and trans caused by missing dowels (for example...) Maybe you've got an engine or trans mount issue or even a pilot bearing or trans input shaft bearing problem.

Give it a few hundred miles to see if it settles down.

I was having pretty bad chatter similar to how you described. From a stop to a roll in first gear on a hill was pretty violent. I replaced both motor mounts (which were destroyed - my car is a 1996) and that got rid of most of the chatter. There is still some there; it might be due to contamination or other damage.

Trinity gave some pretty good tips for diagnosing the problem assuming that your dealer can install a clutch correctly.

A few months back I did the clutch in my 2000 GT. I used a new stock type LUK 11" clutch and resurfaced flywheel.

Used new pilot bearings/TOB. Installed the dowel pins. I did all the work myself so I am sure everything was clean and correctly torqued.

The brand new clutch did chatter a little. It got better after warming up. However, the chatter is completely GONE after a few hundred miles of use. The "feel" of the clutch has also improved with driving.

So I guess it needed to break in just a little.

Something to consider. Drive it easy for a little while and see what happens.

Do not have any more work done at that stealership.