99' Malibu

I have a '99 Malibu, approaching 150K. Last month, I had to replace the catalyic converter. Now, a month later, my engine is not going over 2000 RPM, everytime I gun it in an attempt to go beyond 2000RPM, it flat out dies. I noticed that a bracket broke off of my tailpipe and is hanging lower than usual, but for the most part, still attached. I had it taken to a mechanic, and to my suprise, was told that the catalyic converter was bad again! In terms of regular tune ups, everything is current. But what else could be the cause of the engine not going over 2000RPM? Also, how can that foul up the catalyic converter? And finally, what can I do to keep this from happening again, as it is a troublesome (and expensive) problem?