I think the IAC just needs some time to adapt to all the changes. it seems to be a slow reacting mechanism. with to much air going though it ,it can't adapt fast enough to compensate, which causes the surge. Like Jeff says, with the IAC hose plugged off ,adjust the idle set screw to idle at about 400-500 rpm ,cold . then reattach the hose and reset the computer.This should allow only a small amount of air through the IAC which it can control better. Wait until it is cold again and try it. take it out for a drive and warm it up . let it cool and let us know how it goes.
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