IAC just controls the idle air volume that passes the closed throttle plate. it just controls idle speed when you have your foot off the gas. don't try to make it do anything more than that.
if it stumbles when you press the gas pedal, that sounds like a vac leak. to prove it, LABEL, then disconnect and cap off all vac lines at the manifold. caution, you may not have power brakes during this test. if it runs better, then it proves there is a vac leak, and you can find it by adding the lines back one at a time.
timing chain doesn't cause a major problem unless broken, jumped, or slapping the case. i doubt you have any problem with it other than some wear. i don't think this is your problem. i think its vac.