[blykins]

Check your valve guides for wear while you're looking things over.

Under very high rpm loads, parts will move all over the place. Rocker studs will deflect, rocker arms will deflect, valve guides will start to wear in one spot, etc. The combination of all of these will cause the problems that you're seeing. Everything is dancing around and moving and the areas where you had clearance before may not have clearance anymore.

I'd say your problem will be solved with your shaft mounted rocker arm setup.

I'd pop the valve seals off and do a quick "wiggle" test on the valve guides...