1995 Mercury Sable Poor idle

My Sable has a stutter when you give it gas or if you let off the gas and re-apply it. It also slightly irratic when sitting still. I have a check engine light on and went to AutoZone to have it checked. They told me it had to be 96 or newer. I don't want to just tear things apart and spend a fortune. I don't have alot of money and a diagnostic is expensive for me. Also the check engine light only comes on after it's been in gear a few seconds.