usually, if the cat clogs, it'll idle ok but the more load you put on it, the worse it'll get.
easy test is to hot the cat with a rubber mallet and see if it rattles. if so, thats your trouble. slightly less easy test is to pull the plugs and see if its running rich (usually will if the cat clogs/exhaust restriction.)
'90 YJ, ford 8.8, SOA, lincoln front, aussie rear, 4.88s, swampers.