Why is my 2L GTI idling so low? When I start it its fine for like 15 minutes then it slowly finds its way back to 720Rpm?

Its really good performance for the first 15, then after about ten minutes its idling at 720rpm, I know its cause its warm but I keep stalling it, ive checked the redirection pipe to the throttle body cause thats usually the case but if it doesn't stall it judders.. Somehow the ecu isn't picking up that it needs to idle higher, I haven't got the computer to fix it so its going to stay like that I suppose.. Any idea if theres any common probs that cause this?I haven't really done much fault finding tbh but I know it could be a whole number of things