You may be over mixing the BF, as you suspect. I used to mix it for more than 10 minutes. My dough was kinda tough and not easily pliable.
I recently tried Peter Reinhart's Neo-Neapolitan dough out of "Artisan Breads Everyday" and he calls for using BF at 70% hydration, but only mixing it for about 5 minutes. I wouldn't have expected this, but it was beautiful to work with. Pleasantly pliable. I was very happy with the results on my LBE, and I'll be using his formula for a long time.