Mk1 idle prob

Hi i have a mk1 gti 1600 and when i really rev it the idle returns to a higher position at idle between 1000rpm-1500rmp instead of around 800-900 where it started. Is it likely to be just the idle screw  moving on its own when reving high as its turnable by hand?so vibration or air flow. or is there anything else that could cause that?