Hello, I have a house that contains plastered (contains hair) internal stone (flint) walls. I would like to do the usual fitting of shelves etc but cannot drill into the stone. I have an SDS drill and have just had trouble retrieving a no.8 bit as the tip had melted into a flattened lump! the drill wanders badly too. I have moved from a cob house so hoped the problems would be easier to solve! Is it simply a case of getting a better quality drill bit?