That spring looks like someone bead blasted the rust off of it.

 

You need more seat and open pressure with heavy roller rockers. It wouldn't cause the breakage but it'll show up as a flatline of power at 5500+ as the valves float. I'd swap to stock rockers or upgrade the springs.

 

Curious if you checked the wipe on the rockers? They appear to be biased to the exhaust side of the valve stem but it is hard to tell from photos. If they did a valve job and sunk the valves in deeper than this could explain an exhaust bias.