Check the other threads. I'm having an issue with fan control relays not turning on the fans at all. Normal airflow will keep you from overheating when driving, but idling will start to cook everything hotter with no airflow. CHeck your fans, if they're not coming on when you let your car idle for 10min, then your relay's probably bad. It's a $13 part from mitsubishiparts.net.
WHen did you last get a radiator flush? Do your fans come on when you turn on your AC? Check the tech section and do a search for "overheating" and you'll see a few other threads that troubleshoot this problem.
I did have the cat issue, and it was a recall. Since it's out of warranty, just get a decent header and downpipe from kamikaze. Cheaper than getting another stock manifold/cat setup. Looks and sounds a lot better, too
especially the ceramic one...