1988 Other Chevrolet Models Truck Idles up and down

Engine Performance problem
1988 Other Chevrolet Models V8 Two Wheel Drive Automatic

I have no idea:) The truck's check engine light come on and befor I knew it I was having to feather the gas to keep the truck running, now it idles up and down and befor I did not have to give it any gas to start it. The engine only has 42,000 miles on it. Thanks for your help.