91Z28

Ok here's the scoop, last summer i had both my cats replaced with some high perf. ones, unfortunatly i dont really know who the manufacturer is, anyway i replaced both, not two weeks later one of them went bad, broke up inside, so i took it back, and had another put on. Two weeks after that, the second one faild, also broke up, so again i had another put on. Now just about eight months after putting them on, it seems as if another has gone bad, also broken up. Here's what my car has to offer...

All stock, new oxy sensor, new plugs and wires, oil changes regularly, runs perfect no gas smells until it gets clogged after the converter fails, runs at "Stoiche" 90% of the time, and i never really race it much, i hardly drive it. So what could possibly cause this ?. Every time they have failed it's because of breakup, and i havent hit anything with them, no dents in the converter ?