How close you running to coilbind? You checked your piston to valve clearance? Retainer locks got enough clearance to the seals/guides?

Sounds like an impact issue. Like what you get with too much lash, a collapsing hydraulic lifter, valve float and the "snap" you get when the pushrod ball rattles off the socket.

You got plenty of guide clearance? heat may be causing the valve to float due to stem drag.

I think your rocker breakage is a SYMPTOM, and not the problem maybe.
mc

exactly correct

Yep!!!

Any chance the problem is a qeometry issue, with the pushords the wrong length? I had an engine stuff pushrods clean thru the rockers until we found the stock pushrods needed to be special length. I agree with looking at coil bind also. Guide plates that aren't aligned properly can cause some weird problems also..