I think Carl is on the right track. I mention to check the geometry, from the lifter, push rod, rocker, valve stem and spring. I haven't encountered it very often but you could have SPRING BIND.
With the the valve all the way open and the lifter on top of the cam lobe and the rocker to stem clearance set (valve lash) check the clearance between the coils on the valve spring (inner & outer) 360 degree, top to bottom. Start .001, .002 etc. (between the spring coils) Rotate the engine a few times and check each one.(intake & exhaust) Without knowing all the spec's on your engine, I can't give you the exact clearance between the spring coils. Also check spring tension. I figured you have done that. Good Luck
