I have same problems with my pony... He was perfect (I rode him about 2 years before I bought him) and he started to buck me off... Later my mother heard that my pony was very quickly put under saddle by "a good rider"... But they had found how to do well with him buy making sure that he will have no energy to buck his riders off... When I bought him he changes a lot... now he is beautiful, but sometimes he buck me off without any reason (no pain, no fear), and he trust me so I don't know...
I will try Pat Parelli games now maybe this can work.
But I think your horse is in a new situation, new rider, new home, he is testing you, be sure to stay the "leader".
Good luck, I wish you the best ;)
"Ce n'est pas une liaison, c'est ma raison de vivre"