The problem with clutches is that normally to find a problem you have to strip it down so it is always adviced you fit a new unit while its in bits a bit of a chicken and egg situation.

Has it done a lot of miles or driven like a sports car judder is normally a sign the plate or pressure plate is on the way out or could be oil leaking onto the plate from the crankshaft seal..

You do not say what year or model but clutch judder seems to be very common with the MX5. Check out the MX5 owners club forum, there you will see many examples and the answer seems to be a replacement clutch not made by Mazda.

Have you tried just raising the rev's slightly when pulling away?

It may be something that appears quite unconnected, for example a worn cv joint, or if kangarooing, a weak distributor or dodgy O2 sensor.

If it's just a little quiver of excitement, aren't MX5s supposed to do that?