S.S.
We went through this with my son when he was 2 1/2, and our ECFE teacher referred to it as "functional dysfluency." She described it like you did, where he had a "Ferrari brain and a Matchbox mouth." In other words, the latter couldn't keep up with all the thoughts running through the noggin! She explained that it's something that comes and goes for various reasons, and that in MOST cases, it would pass on its own. She also said not to focus too much on it, like saying "slow down," which actually frustrates them more. It did pass, and came back a year later for a couple weeks, and then we haven't seen it since. Here's a link with some signals for seeking further help. Good luck, I know it's frustrating!
http://64.233.167.104/search?q=cache:GUgS-ZLL0rEJ:www.lan...