Cold start problems

Still having problems with my 1.4 16v when my engine is cold, the car starts fine but after about 10 seconds of tickover, the engine starts hunting badly and jumping from arround 500 revs to 1200 trying to keep its self going, ive replaced the coolant temp sensor as directed by people on here but the problem is still there, today i was cleaning my engine bay and had the engine cover/airbox off, i thought id try start the engine (from cold) see what happens, no idle problems what so ever, this now leads me to think its the hot/cold flap in the airbox that isnt opening or closing properly and starving the engine of air.

could anybody shed any light on this for me please? cheers weedon