I have been reading on the PSSM Forum on facebook and it sounds like some of the issues that members talk about occurring with their PSSM positive horses. I gather that when the horse is experiencing symptoms, they are in pain and can express it as bad behavior. The symptoms are not always there from birth, sometimes when a horse is older it becomes symptomatic. There is a genetic test using pulled hair for PSSM1. You might want to check out that forum since you have already had a chiro and vet check. Just another idea.