Check all vacuum and vapor hoses and make sure they are all there and in good condition. Ihave an 87 with the 4.2 and it was missing the entire pulse air system. It won't idle right until ity warms up and is failing emmisions. WE are trying to locate one in a junk yard as they are no longer available from manufacturer. Other than that check timing and Carb. Griz
I upgraded mine to fuel injection to cure the same problem. Not cheap, but solved my problems. Yours is already injected so that won't help. You might want to change your plugs...all the norman tune up stuff.