I joined my school at the same time as my LM, who had previously been HoD in my subject, and had similar experiences to you (presumably also similar to you was the fact that she was teaching IN the department, too). Patience was the key thing! There were a couple of times when I actually had to admit that her ideas were better than mine, but it took a while to see through the red haze. There were also a couple of occasions when we clashed to the point that I got the Head involved, and I think the Head did have a word with her to let go. So, if you can endure things more, try to do so; if not, speak to the Head.