Skinned animation problem

"My character's run and punch animations look fine. But his kick animation has a split second where he looks really distorted"
Nobody will tell you what's wrong on the basis of this description:D

"I think it might be that I'm using linear interpolation with matrices"
For rotation use spherical interpolation of quaternions, for position linear interpolation is OK.

Yeah, I may have to switch over to translation/quaternion approach but I seem to have fixed the problem by renormalizing my matrices post-interpolation.

vectors can become un-normalised when interpolated. A matrix is just grouping of vectors, as you found out its always a good idea to renormalise.