Yeah, I have the same thing. Its common and pretty interesting. I need to order a medical prescription every so often. It's done through an automated phone service where you give your name, address, dob and the name of the medicine you're ordering. I have a really distinct stammer, yet when speaking to the automated service I can speak fluently. I'm sure there's been academic findings recently as well which say the same thing, that stammering only really comes on when speaking to another person