I've been there. Have you considered contacting the employer in advance and letting them know you have a stutter? This might take away some of the nerves and help you to perform at your best. I'd also encourage you to give the employer the benefit of the doubt. They might not see the stutter as the problem, especially if they are aware of it before the interview and you have been assertive enough to declare it. If they are a dick about it then at least you know it is not the right workplace for you.