1990 Chevy Lumina misfire

Engine Performance problem
1990 Chevy Lumina 6 cyl Front Wheel Drive Automatic

Hello, I have a 1990 lumina euro(3.1MPFI) with 86,000 miles and recently it has been running poorly, misfiring. I just changed the plugs and wires yesterday, about 6 months ago my car kept stalling I had replaced, fuel filter, ignition module but it ended up being a fuel injector that went bad and I had replaced. Now the engine is misfiring, I was thinking MAYBE it was another fuel injector, but it's not quite the same symptoms. I am thinking maybe I have a clogged catalytic convertor. My muffler rusted off about a month ago and the whole exhaust is shot. When I drive it since it started misfiring, the engine sputters and bucks. Just runs very poorly. When the misfiring symptoms first started, it would run great until about 5-6 miles when the car was warm, it would then start running poorly(misfiring). Now after changing the plugs and wires it runs poorly all the time even on cold start up. So possibly a clogged convertor? Ive been told that the convertor is broken inside a few months ago, just dont know if thats what would make it run poorly and misfire. Thank You