I've got a 2002 mazda protege 5 speed manual with a bit over 105,000 miles on it and i believe the first clutch. After the car gets good and hot, it shutters (vibrates the hole car violently) when letting the clutch out in first gear. I am absolutely sure I am not lugging the engine because it does the exact same thing when other people drive it. I noticed a while back the the front and passengers side engine mounts were ripped and thought this to be the culprit. I changed them and it has become somewhat less violent. Not quite sure what to try next.