Walking straight backwards with "s" causing animation to stutter

Hi,this is just what i need. But my bp is different from this one and i don’t know what the node is before <=. When the direction is less than -179, i make it +360 and that is all i do. The problem is, when I press S and make the character walk backwards, there will be a obvious shake on him at first and everything is normal after a while. I guess it is the problem of data import.