New, heavier valve springs perhaps? That would stress any old parts in the system... valves, rockers, pushrods.
Valve broke, dropped into cylinder, hit piston, bent and jammed in guide is what comes to mind. If the valve is tight, then the head needs to come off.
Time to pull all the pushrods and roll them on a flat surface to check for straightness.
How much were the heads milled?
