Not that this is always an excuse for bad behavior, but did you consider perhaps your saddle might not fit well? Especially since you say you are sliding around in it? A well fitting saddle should fit well even with a loose cinch.
Just a thought. It would explain the grumpy/cinchy behavior and the saddle moving around.
I had a Arab gelding that used to be crabby, cinchy, and would bite. We all just excused it as his disposition, but looking back on it I'm sure his saddle never fit well at all. I was just too young and green at the time to realize it.