Hey GEC52, You mentioned its #6 again, have any other ones broken? If it is only #6 here is a theory I have come to. The valve stem or the guide is either burred or slightly deformed or bent. What happens is that the valve will turn slightly as the engine runs a while and this causes the valve to lock intermittantly when it does it breaks the rocker.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"