That screams to me of an issue with your setup rather than a code problem unless you've done something weird, I remember when playing around with a vehicle controller in Unity and having that issue a lot. Usually what causes a lot of physics weirdness like a rigidbodies flying in the air or flipping the moment you start is there's some sort of overlap happening and yes, this can be the case even if it's overlapping by a miniscule amount, game engines just don't like that and freak out.