When spokes break, have you had anyone check and true the rim and tension all the spokes evenly? Not just the one that needed replacing, but all of them? Sounds like the rim is too bent, or spokes have widely differing tensions after changing single spokes many times over.

Also, do you lose spokes consistently on one side of the wheel (drive side = the side where the rear gearing is, or non-drive side)?

--J