I really find this a bizarre problem. I've experienced one condom break in eight years of sex, and that was a very freaky accident.

The only thing that springs to mind is whether even a small amount of some substance that eats through condoms could be coming into play (even through brief contact of the condoms with your hands or your girlfriend's), and gradually eating through the latex ---- such as residue from baby oil, nail polish, cleaning fluids like methylated spirits... perhaps even some other form of cosmetic scent product or the like?