We are all learning, that's what fabo is here for. I'm trying to throw questions out there that may lead to a solution.
IF a valve is hanging open....wrong pushrods?? Valve guides too small(grabbing the valve?? Coil bind as mentioned?? Valve height too tall?
You say heads rebuilt....pretty broad statement....new valves/guides/springs? Or were they transferred over from previous heads?
I'm hoping to learn from this as well.