Valve stems might be sticking. This happened to me. Shoot penetrating oil down the valve stems on top of the valve guides. Also, remove the side cover and rotate the engine by hand and be sure everything is moving correctly (might as well spray a little penetrating oil around the mechanical lifter bodies, too).
I ended up doing what HRL suggests above - take it apart and take the head to a shop for reconditioning. They will most likely tell you the valves are bad and the valve guides are out-of-spec. Try to get the valve guides replaced or sleeved - knurling the guides is a weak plan-C in my uneducated opinion.
