Hi Micha...
Oh oh... I really feel sorry for you right now. It's probably definitely the thermostat. You're going to have to 'strip' it, meaning remove it, from the front... it's located relatively low down at the front of the engine, next to the water pump. The castle support must be removed, and the holder for the viscous fan must also be removed.
I recently had to replace my power steering pump, almost the same procedure... it was a real pain

.
Well... have fun.