Why do I keep breaking rocker arms?

I still haven't seen any thing about the cam specs, or valve springs. Or whether the head had any valve guide work done to it.
I have run cams from only .434 lift up to .528 lift. Used 340 style valve springs, and have never broken a rocker arm. Even turning close to 7,000 rpm (170 cid). However in most cases, I did have to machine the valve guide, and/or spring seat for proper clearance.