Motorcycle Repair: Keep blowing rocker gaskets, stock air, error approach

Question
I've got a 99 heritage. About every 100 miles rocker cover gaskets blow, mainly rear jug, as I can tell. I'm assuming that engine isn't breathing right. How can I determine weather it may be the breather gear, cloged breather tubes, or something else, with out spending tons of money with trial and error approach. Bike has stock air box with K&N filter. Thanks

Answer
First thing I would do is remove the rocker cover and check to see if it is warped. They have been known to do that. If the bike wasnt breathing right, both front and rear would blow. Your breather gear runs directly off the cam, so it can't be off.
Good luck and happy riding
Mike