1998 Mazda 626 catalytic converter

For the third time in less than a year, the 'Check Engine' light on my Mazda 626 has indicated a problem with the catalytic converter. The first time, last September, I had my service shop (which specializes in Japanese cars) replace it, at a cost of about $350. By January, it had fouled out and the light went on again. The shop replaced it under warranty for free. Now (July) the light has come on again--the same shop is replacing it for free, under warranty, with the highest quality CC they can find, but they want me to sign a release for no more free replacements, only a refund. Before the first replacement, I drove the car (which I bought used) for 3 years with no apparent CC problems. What's wrong? Can I do anything to the fuel mix, etc, to prevent losing this CC as well?

Thanks.

Joe Jennings
Houston TX