Let's look at other areas - where are rockers breaking: towards one of the ends, or in the middle?
Are you seeing any unusual wear on the rocker arms where they connect to the head? Check the unbroken rockers for signs of wear.
Are you running a stock camshaft, springs and valves?
Have you confirmed you're getting oil to the rockers?
I can't imagine a rocker breaking unless it's sticking, holding the valve open, and having the piston smash back up and close it.
-Boyo
