Why is my horse bucking and what..

Brief：
He is a 6 year old thoroughbred gelding. I bought him about 4 or 5 months ago from someone that had only had him a week. Before that, he was on the tr.. 

He is a 6 year old thoroughbred gelding. I bought him about 4 or 5 months ago from someone that had only had him a week. Before that, he was on the track. I was the first one to ride him since he had been off the track and besides being a little quick, he was a perfect gentleman. I rode pretty consistently for a while, and then not so much for about two months because of school and finals. When I started riding again, everytime we picked up the canter he would buck. I stopped him firmly and told him no. This continued so I lounged. He doesn't buck on the lounge line, but when I get back on he bucks. I have now ridden him HARD 6 out of 8 days and each day he has bucked. I changed saddles, put a traumatech therapy pad on, changed bits, had his teeth checked. I'm not overweight at all... 5'4" and 115 lbs, and I have been riding and training for over 10 years so it's not like I'm unbalanced and all over the place. Please help, I don't know what else to do except sell him. I hate buckers!

If you have ruled out pain then my guess is he is either doing it to get out of work (you said you make him stop = BREAK TIME!) - or just high spirits. I have had a couple of horses that have bucked from sheer exhilaration when they broke into a canter.

A lady I was taking lessons from advised me to do quick transitions - preferably before the buck happens - trot-canter-trot and change of directions too. The idea is to keep the horse focused and having the horse expecting that a transition is coming up so don't waste energy bucking.