Rough Idling and rough start-up

For about a year now my 2000 Ford Explorer with a 4.0 V-6, 112000 miles has had rough idling and very rough starting (at times will not start). The check engine light has come on numerous times. The first time I brought it in to the shop and they cleaned the fuel system out with the checmical solution stuff. That adventually led to a jammed plunger in a fuel injector on cylinder 3. So, in turn I got all 6 fuel injectors replaced. For awhile it ran fine and then the check engine light went on again and rough idling/staring persisted. This time I brought it in and they replaced the spark plugs and spark plug wires. It ran good for awhile again, but now it is back to rough idling and starting yet once again. Help. (FYI I have also noticed that when I add fuel system cleaning products the engine begins to run worse).