That's interesting. Do you stammer more even when speaking some English you know well (ie when saying a simple sentence such as "Hello. My name is ...") or is it worse when you are trying out new words and constructions?
One of my students in Spain stammered only when speaking her native language. When speaking English, her stammer vanished. It seemed to work along the same lines as people whose stammer (or even Tourette's syndrome) goes away when they are singing.