Catalytic converter issues

I have a 1995 BMW 740il, american spec. I am station in south korea. I replace the converter with after market ones, plus new oxygen sensors. I assume they would be o.k. I see them on every online part store. about two month, both converters when bad. I thought maybe I had a bad brand, so I order two more of a different brand. Now they are starting to raddle and my car has lost power. This is the second time I had to replace cats less than a year. Do you have any clue on what could be causing the cats to continue to go bad? I am ordering new converter plus oxygen sensors, but not sure on where the problem really is. CAN YOU HELP?

Try the Magnaflow sports cats. They are supposedly durable where the standard ones are not. I seem to remember that California "certified" ones are supposed t6o be more durable also.

I've got Magnaflow cats on mine w/o a single issue. There installation is critical as they can't be moved too far forwards or backwards from the original locations where the stock ones were mounted or else you'll get "cat efficiency" codes.