Just one more question: is the spoke itself breaking where it threads into the spoke nipple or is part of the nipple staying with the end of the spoke and the 'head' of the nipple breaking off in the rim? If it is the latter, it's very possible that the wheel could have been laced with alloy nipples instead of brass. Alloy nipples do not stand up to severe service - - and I would put clydesdale rider in the 'severe service' category.