Lope at idle

I have a 1994 chevy s-10 blazer, 4.3L vortec v-6, 152726 miles. I just bought the truck 3 days ago and it ran fine. The next day it started to idle a little low and lope during idle. Also when you mashed the brake it would idle up for a second or two then lope again. To get it to crank you would either have to just hold it over or pat the gas a couple of times. When driving at normal speeds it would run fine. But when you stopped at a light or stop sign it would lope and try to go dead (sometimes it would die). Today I replaced the plugs, plug wires, distributor cap and rotor button. Nothing changed. My friend at the parts store said it could possibly be the catalytic converter, the idle/air control valve, or the throttle position sensor. He told me to pull the idle/air control valve and clean it with carb cleaner to remove the deposits from it and that might work. Did nothing at first but then kinda smoothed out a little but still idles a little low but not near as bad. Could it be the cat converter (hate those darn things) or one of the sensors my friend metioned. Also will my blazer run ok without the cat converter?