I have an '86 GTI when first starting after cooling down it doesn't want to ifdle until the car warms up , then it has a high idle.
I change the throttle body with one off an '88 golf but still have the same problem
does it idle at like 3000. If so then try unplugging your isv and see if it goes to normal. If so try cleaning it out with throttle body cleaner. If it still does it replace it. Mine did that and the isv fixed it. But if you do get one used because they are almost 300 dollars from the dealer
Keep us posted and let us know what you come up with