Just installed a spec stage 1 and every time I start from a stop the whole car shudders horribly, no matter what I do. Shifts fine otherwise. So far I have put about 15 miles on the clutch.
Input shaft bearings, pilot bearing, the retainer the throw out bearing rides on, the throw out bearing itself and the flywheel surface. Any one or combination of these can cause this.
Did you do the work and what didn't you replace?
I replaced the input shaft bearing retainer with a steel one, I replaced the pilot bearing, the throwout bearing, and had the flywheel resurfaced. I did not replace the input shaft bearing.
You wouldn't have as a rule unless the trans was getting a rebuild, did you put a little grease on the cone if the retainer?
another possibility would be a sticky spot with the cable.
I still would suspect the flywheel or pressure plate. And please check the cable. After breaking one, I include that when replacing the clutch.
Greasy fingerprints are the only other thing I have not seen mentioned that is even close to a common cause I can think of.