I know for me personally, my stuttering is related to my racing thoughts. If I take the time to slow down and concentrate on what I am saying, or plan out what I am about to say, I can get through sentences (And presentations) without stuttering, but if I get excited, or if something throws me off, then I starts to stutter over my words and then get more nervous because I am stuttering and it gets worse. I usually have to stop completely and recompose myself before continuing to get it to stop. My wife didn't realize I had a stuttering problem until after we had been dating for a long time (We dated for 4 years before getting married, and I believe it was year 2 before she ever heard me stutter).
I can believe that blood flow to the language centers of the brain could cause stuttering as this area then can't keep up with the rest of the brain and then the person stutters. I know mentally that this is the cause of my stuttering, but it is beyond my control unless I really concentrate to contain my thoughts and slow myself down.