My 2001 Miata has a chatter in the clutch when the car is first driven, especially on a cool day. The clutch never slips and shifting is excellent. When it starts to chatter it will get quite bad unless I drop the clutch or rev to 3-4K and feather the clutch to stop the shaking. What do you recommend?
If the clutch has a lot of miles on it (you don’t say) I’d suggest the damping springs in the clutch disk or the marcel spring between the disk faces are worn out. That means you may need a new clutch.
If it just does this early in the morning, the clutch material may be absorbing a little water overnight. Once a little heat gets in, it drys out and all is OK.
Another possibility is that is isn’t the clutch disk at all and a bushing in the rear suspension has worn out (again, you didn’t tell us how many miles - but it IS 18 years old) and when cold, sets up a resonance with the clutch.
Mazda had a recall for clutches that did this for the early 2002 Miata models. Whether the same problem exists in your car I don’t know, but the symptoms sound exactly the same. Humid weather turns cold starts into a shuddering, lurching mess. It’s not a safety recall, maybe better to call it a technical service bulletin that they would replace a bad clutch under warranty.
According to safercar.gov, Mazda put out three manufacturer’s communications for this issue. See if you can find 0500103, 0500202, and 0501202. While often available at the website, these are not. Do a web search or ask Mazda.