I have a '98 SE with 33K miles, and manual trans. When the ambient temperature was in the 30's or colder, I would get a severe chatter when first starting out in the morning. The idle would be up, but it seemed that giving it a little gas would cause the chatter. It also seemed to be worse when then turning left from a stop. After warmed up, it would run fine. Now that the ambient is high, I haven't experienced this problem. I'm wondering if it might be too lean a mixture at start up, or a major problem with the tranny. I'm wondering if anyone else experienced this problem.

Is this chatter evident only while your foot is on the clutch pedal? If so, you have ordinary garden-variety clutch chatter. There's nothing wrong with your engine or transaxle.

My 98 SE has only 33K kms (20K miles) on it and, as I've posted before, the clutch engagement is very non-linear when the car is cold. I live in the city and the car sits in an underground parking garage almost all the time and it is a little jerky to drive for the first couple of minutes after starting. However, when I am out of town and the car is parked outside all night, I notice significant clutch chatter. As I first pull away (when the car is cold), engaging the clutch a bit slower than normal to avoid being too jerky, I notice significant clutch chatter. This goes away after a couple of minutes and the clutch then operates smoothly. This seems really weird to me. I know that others have reported chatter, but does everyone else only notice it on start-up. Is it common on low mileage cars? (I don't abuse the clutch)
Daniel, maybe you can answer this, what is causing this chatter (that goes away after warming up) and is this a symptom of a future problem?

Chatter is a rapid slip-grab-slip-grab phenomenon. The clutch engagement is "jerky" instead of smooth. Clutch chatter is felt more than heard. It occurs only while the clutch is engaging, and is most noticeable when starting from rest. As soon as your foot is off the clutch pedal the chatter stops.

Clutch chatter may be caused by, or aggravated by,
- oil or dirt on the friction surface (leaking rear main engine oil seal)
- glazed friction surface (excessive heat resulting from abusive driving practices)
- pressure plate diaphragm spring misaligned
- loose or broken engine mounts

It is not possible to say if this is a symptom of a future problem. That is because we don't know what is causing the chatter in your particular car. If it is caused by a leaking rear main seal, it will get worse with time.

The hypothesis of a leaking rear main seal is consistent with the change of behavior with change of temperature. The traces of oil will change viscosity as the clutch heats up.