Go to another dealer to get this dangerous problem fixed then complain to whoever will listen within the FIAT organisation, that really is pants...
that has happened before on a new 500 seems theres a bad batch of cylinders as they are normally fine used in many a panda also and punto and the cylinder in that case was replaced under warranty and all was fine
it looks like who ever road tested the vehicle didn't notice it as it's intermitent and simply disregarded it assuming you can't drive i have seen it before
that is the most unacceptable thing i have heard on here to date get a complaint in at fiat and your tail gate if i were you especially the clutch that's very dangerous as i say it will be the master cylinder
have you tried sitting in first with the clutch in for a length of time with lots of room infront like on your driveway for example see if it does it