I too am at a loss as to what the problem is.
With the information given things just don't add up.
About starting in a higher gear, my clutch will actually chatter more. I usually start off in 3rd or 4th, but if I mistakenly start off in 6th the clutch chatters, or at least the dampener springs in the center of the disk act funny. This only while trying to release the clutch, if I go ahead and let it all the way out the engine try to die, but keeps going just under a normal idle, and no clutch chatter. I do have reasonably solid mounts, with about a 35" spread between the rear pair.
I would read your service manual and adjust the clutch peddle as it tells you. As the others have said, you can't actually adjust the clutch, only where the peddle and throwout bearing are in relation to the fingers on the pressure plate.
Check your timing and vacuum advance, I have an idea that I can't seem to put into words, but for some reason I think bad timing could cause the symptoms?
Grigg
