Head Issues When Character is Running

It is a bit hard to see, you mean the neck and head bending in ways you did not expect? I fiddle with the location of the head and neck tags myself. I tend to put the neck tag at the base of the neck (on a handle) and the head tag up closer to the bottom of the head. Adjusting their relative position changes the body posture up there. I don't have an exact reason why, I just fiddle and play and see if I can get it to look better. He sometimes stoops more than expected when they are not "right". Also the Strength properties of the walk behavior can help sometimes too.

I don't use the walk behavior very often though, so never spent the time working out good debugging practices. But its usually relative position of handles that makes a difference. Shoulders, waist, hips, etc. I try fiddling until it looks better. (Oh, the yellow mesh view (icon at bottom of scene window) sometimes helps as well - if it shows a strange distortion at a spot that gives a clue which handle to try moving next.)