hi mate, my old s2 used to do the exact same thing, it was(belive it or not) low oil pressure on idle causing the engine to "hunt" (rev up and down) aparently the lifters were compressing and not letting some valves open properly, so when it was cold the oil was thick and pressure was good and idle was good but when the oil warmed up it went alot thinner as do all oils thus causing the problem,

or it could always be an air leak...

idle control valve leaking or knackerd.

worn camshafts can cause hunting.

and depending on what fuel system and control you are using...your cold start valve/injector may be playing up.