that idle screw isn't like the idle screw on a carburator... i think that screw is more to provide the exact stop where the butterfly comes to a complete close. turn that screw too much and what you are doing is partially opening that butterfly - to do the job that the IAC is supposed to be doing. I say you are adjusting one part to compensate for another part that is misadjusted/bad
when that throttle plate hits the tip of that screw, the butterfly should be *completely* closed - I don't even think that screw would be classified as an 'idle screw'