Wow… lots of assumptions 
He was started in his 4yo year, rode him that whole year until about November. Then worked him on the lungeline. Started riding him again this year in July or so. Horse has been under saddle for 18 months total. He is in great shape and the work is easy for him. Really…2nd level is not rocket science 
Then… yes he had some young horse problems, vet came out last year and horse was thoroughly examined, everything was checked out and he is healthy. And really, none of these problems exist today… So not so sure how that is relevant now.
The buck, is one buck. Only when I pick up the reins the very first time. If a horse is in horrible pain I would expect other behavior…like, oh, a whole bunch of bucks? Sour attitude? Inability to collect?
Besides, the vet and the chiro have been out, as well as the saddle fitter and the farrier. This is not a horse in “horrible pain”. A horse in pain would do it on a trailride too, he doesn’t.
Mostly the buck just annoys me, it’s not exactly scary
But I would like him to not do it, just in case someone else ever rides him, or if I would ever have to sell him or whatever.
I always stretch the frontlegs, he has never fallen down yet 
I did wonder if it could be connected to the girth somehow though… I have a very simple Wintec straight “plastic” girth… maybe I should try a different one?
I have never ridden him bareback, but I will try that too. Will try without the bridle also. And riding him on contact from the start 
Hope I answered all the questions!