Why after doing a spark plug change will my car idle at 1000rpm

Hi thanks for that did that plus changed the leads as i noticed the rear spark plug leads were a different make and looked a lot older...ok now it purrs like a cat trouble is at 1200rpm if i remove the lead to the iac it returns to normal revs if i turn the a/c on it increases revs as expected turn the a/c of back to 1200rpm...put it in gear idles normal...put the lead back on the iac 1200rpm ...also sometimes if i increase the revs in idle to 2500-3000rpm it shudders and hesitates then as the car warms it is ok and will rev to 4000rpm without a problem but then agin to 1200rpm idle...could the iac valve or egr be stuffed both items are original....Thanks