I have a 57 Corvette which has just undergone a complete restoration . It has had a (new flywheel from Summit Racing ) and clutch kit installed and it was so rough starting out that I tore it down and replaced the clutch kit with a more expensive McCleod clutch kit . The second clutch kit still did the same thing but it was with the same new flywheel so I started to suspect that the flywheel was defective since there were hot burn marks on one half of the flywheel and pressure plate so I took it to the machine shop and had it checked and refaced but when I put it back in and tried to drive it the same problem existed . I have done everything that I can think of including new engine and trans mounts , checked all clutch linkages , checked to make sure the throw out bearing is not binding , universal joints , etc . Everything checks out good . Now my question : I have a posi rear end and it was making a shutter going around turns so I changed the gear oil and put in the additive for posi but no work was done to the rear end during restoration and I can hear a bumping noise when I back up . Could there be a problem with the posi unit in the rear end making it seem like clutch shutter ? I can't see how there could still be a problem with the clutch . It drives smooth when driving down the road and shifting while moving . It just wants to shake the car apart when starting out . I even thought about an engine balance problem but the engine runs smooth . I've been working on cars for over 50 years and never saw anything like this problem . Any help would be appreciated .