the catalytic converter doesnt block any cooling passages. If it does have build up the car will feel a little powerless. If it gets bad enough it wont run hardly at all and go into LIMP mode where you run on 4cyl. and have little to no power then. Is there a check engine light on? Pull the codes if you can. the auto parts stores sometimes have the tool and do it for free. If the carhas had the headgaskets replaced along with all the other parts and still over heats you may have cracked heads. they should have checked this when doing the headgaskets so im going to assume you dont. Also check to see that the radiator fans are working that will also cause the car to overheat especially when in town in slow traffic. Let us know what you come up with and well go from there.