My old stock engine used to overheat on the highway, but was fine around town. Replaced virtually the entire colling system, nothing helped. While installing a dual exhaust system, we cut out the cat. It was almost completely blocked by a gummy tar looking goo. There were no rattles in the cat. In my case, the only way you could tell the cat was clogged was by looking into the inlet. Finished up the exhaust system (w/o cat) and it never overheated again on the highway.