You need to get your FC up into the maintenance range of 8 to 15ppm with your CYA. Once it's there, you should have no trouble maintaining it in that range and not be using bleach or LC by the boat-load. In fact, your chlorine usage should go down substantially--but you MUST maintain the higher levels--aim for 10ppm. If you still are using so much then shock it all the way up to 25ppm (as per the "Best Guess" table) and that should kill whatever's eating your FC.

You CAN live with the higher CYA but you cannot do it with an FC below 8, minimum.

Hope this helps.