[Wayne 962]

That's almost 100% bull (in my opinion). If your car is operating correctly, the mixture going into the cat will be the same regardless of the load placed on it. The mixture is controlled by the Motronic engine management system, which dynamically tunes the car for a variety of conditions, based on feedback from various sensors. If one or more of the sensor (like the o2 sensor) is bad, then the car will run rich or lean.

What you really need to do (instead of listening to someone 'guess') is to take the car to a friendly shop that will allow you to check the mixture. This will tell you firsthand if the car is running rich.

CATs don't typically fail on their own - they are usually hurried along by a rich-running car that dumps soot and raw fuel into the CAT. This is what causes the cat to clog. Placing a new cat on the car without fixing the original problem will lead to premature failure of your new cat. The cat should be the last thing to be replaced when you've done repairs, because if the mixture is bad, you will ruin your new cat...

-Wayne