1992 Audi 100 Overheating

This car overheated while driving on higher speed country road on a moderately warm afternoon. (Coolant overflowing the little tank on the side.) The engine began to lose power at the same time. Once it cooled down, it was fine but then overheated after ten more minutes of driving. Once into town, driving slower, it never overheated.

I put in a new water pump over a year ago and the car has been trouble free and great on long trips. Here's something I'm wondering about. It has a separate exhaust for each bank and one of the catalytic converters makes a LOT of noise, as though things are loose inside. Can this cause the engine to overheat by backing up exhaust under high rpm? If not can you suggest how to trouble shoot my overheating problem?

Thanks a bunch!
Steve