5000k + per year, pas mal, but perhaps there had been something in its past...who knows. I certainly dont have the knowledge of metals to know more, but one would think that something had weakened it at some point. I say just from the angle that (to my knowledge) this is not a common occurance.

octave changing indeed. rather disconcerting.

depending on your weight, you could ask in the section for heavy riders, or at least for some suggestions for posts from folks who weigh more than you, just for your piece of mind in buying another.