2005 Smart Car keeps juddering.

I have a smart forfour passion semi automatic year 2005 and it sometimes judders when I start off and most of the time when I'm driving also sometimes randomly it just cuts out and stops and I would either need jump leads or to give it a push the juddering and cutting out is quiet unnerving as it could happen at any point of me driving, please can you give me some advice on what it could possibly be.

Thanks