Topic: Sputtery tongue

I have a student with a tonguing problem - when he tries to single tongue at even fairly moderate speeds his tongue just goes kind of crazy and sputters so we get little bursts of very fast notes and they are not at all even.  We have talked about keeping the airstream constant, and about keeping the tongue small and close to the reed, and have worked on slow-motion articulation, but the problem persists.  Does anyone have a suggestion as to how to solve this?