Get a pro trainer who is accustom to working with horses like this. Like Keven said. Unless you can ride it out you will only end up getting hurt. There are trainers who specialize in training horses who buck. They are very good riders and can ride it out.
I will ask though. Have you check back problems saddle fit and such??? This could also be a problem.
-I'm so busy... I don't know if I found a rope or lost my horse.
-An Armed Man is a Citizen an unarmed man is a subject.
-Where ever free speech is stifled Tyranny will reign.