rough idle

My 1981 Chevy Monte Carlo has been running very rough when idling. When I put it in drive it gets a lot worse, but at high rpm it is smooth. I’ve just replaced all of the spark plugs yesterday and it was running smoother than ever. The engine has 72,000km on it and has brand new oil, air and...

Now before I explain the problem I admit yes this is a J-Body GM, but I have 2 other G-Bodies and have had good luck with this forum in the past. The car in question is a 1988 Chevrolet Cavalier Z24 Convertible, with a 2.8L direct fuel injected engine with a little over 106,000 miles on it...