Hi Melo,
Welcome to the forum. I am unable to offer any help but thought i would give you a free bump :)
Couple of questions, these may not be relevant but it may help to narrow down the possible cause.
Do you have Climatic aircon (manual dials) or Dual Zone where you have an LCD display than shows the temperature.
How is your refrigerant - has it been changed in the last 2 years?
Is there a clicking sound from behind the glovebox, if there is it could be the flap motor playing up.
And on a more serious not. We need photos of your GTI