1999 Ford Contour Rough idle

Hello All,
I am wits end with my car. My check engine light came on and the code was lean engine. The idle started running really rough and my engine died a few times. I have replaced the wires, plugs, tensioner because it was almost falling off, the valve cover gasket it was leaking. But the strange thing is it will idle normally when I turn on my A/C. I was told to check everything from the o2 sensor to the idle air control valve and the egr. I rather not spend anymore money then I should on this problem. I have no vacuum leaks. Any suggestions from anymore that has had this same problem or a mechanic that knows what this could be. Thanks!