Most definitely rule out any pain issues. There are so many reasons as to why a horse could be throwing their head. Have you tried lunging the horse fully tacked with thwarting bridle on and a halter underneath to see if she/he is doing it with no rider? The saddle could be pinching, girth too tight, pad rubbing, also try loosening the headstall by a hole. Some horses don't like a tight bit but a looser one. Also are you using a 2-piece or 3-piece or even a chain mouth piece? Nose band? Tie down? If your using a 2-piece try a 3-piece with rollers or just a 3-piece. I own one of the most ridiculously picky sensitive horses out there. Can't use an off brand bit, nor a 2-piece (2-piece have a nut cracker effect and could be hitting them on the roof of their mouth). Also the bit can't be where a normal bit rides as he will throw his head. Do you float once or twice a year. Some horses need twice a year for the speed their teeth grow.
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