valve stem

Recently rebuilt .030 over 302 Ford to put in my 34 Ford pick up. Have hour meter in truck, engine has ran 64 hours. The other day, lifter started making noise,lost cylinder. Took off valve cover, readjusted valves, still had noise. Pulled rocker arm, discovered top of valve stem scattered, and rocker arm riding on spring. Anyone have this experience before? Valves were done at time of rebuild, has good compression in all cylinders, 60 pounds oil pressure, and all rockers have plenty of oil. Does anyone know what might cause this to happen?