1984 Chevrolet Camaro

Engine Performance problem
1984 Chevy Camaro V8 Two Wheel Drive Automatic 130k miles

I had the catalytic converter replaced on my 84 Z28 about two weeks ago. Recently i've been experiencing low idle at about 500rpm with a noticable shake coming from the car. It hasn't stalled but the idle is rough as if it feels like its just about to though. On initial start up the idle rises as usual but after about thirty seconds the idle drops and the Check Engine light comes on. In top end the car runs like a champ so I'm only experiencing this when parked or at a red light. I called the muffler garage and they suggested that the new cat is already clogged and that it its because I opted for a universal car rather than a direct fit. This just seems alittle far fetched to me that it could malfunction this quickly and cause such a problem. Any suggestions as to what may be causing such an improper idle
July 5, 2009.

Try cleaning out the idle air control valve and EGR valve and see what happens

Jul 5, 2009.