I believe what you're talking about is the "dog-walk". Its their leisurely slow walk, before the flat walk. My TWH has a more "exaggerated" walk then did my older QH/Morgan, which led to, as you describe, more undulation.
I'm not sure why some TWH's feel this way, maybe their conformation, their way of going? I personally don't mind it, but to each their own. I find if I keep him mildly collected it gets a little smoother. Check out thes videos ( not mine ) and see if that's what you think he might be doing. The only examples I can find of the apparent camel-walk ( never heard of it ) are a faster pace walk.