Most planers that adjust depth by moving straight up/down have some sort of feature on the shoe that guides it in the body as the shoe goes up and down. On the 2003 Hitachi, this feature is that raised lip at the back of the shoe and it's not a very positive guide. If the depth mechanism isn't really smooth, it will try and rotate the shoe rather than move it up and down. The best you can do is bond the foam dust seal to the shoe (spray adhesive) and see if that helps guide it. First try adjusting the depth mechanism per the Clark-Hitachi manual; there's an assortment of shims and such which may be incorrectly installed.