Have you ever tried disclosure, ie. telling the interviewers that you have a stammer upfront? I find it really difficult (ironically because I often struggle to say the word 'stammer'), but when I have done this, it's really helped in stressful situations like your interview scenario. Subconsciously, it takes a lot of the pressure off, and even if you do stammer, the people on the other side of the screen will know why.