[Rick Parker]

If you sincerely believe this is happening and have checked your dimensions accurately I would recommend you cease running the engine. You have a catastrophy on the verge of happening. It is possible that you have the incorrect retainers and keepers meaning they are not matched to each other. If a set of 7 degree valve keepers are used with a 10 degree retainer it is possible there could be movement as you describe. If it continues (and there is no reason to believe it won't) a valve will drop into the cylinder and be impacted with the piston and all kinds of nasty, noisy, loud, EXPENSIVE things will happen. If you are unfamiliar with this get some professional help. You may be able to remove the rocker arms and retainers without removing the heads or it may be decided the best corse of action would be to remove the heads, get some advice.