this sensor in this link is the only sensor i can think of coolant related that are common to go bad in mk4's heres the link it causes simular problems when they go bad as your having the mstock one is black i believe or brown cant remember and even if this isnt the problem replace it anyway it needs to be done

http://www.mjmautohaus.com/catalog/p...oducts_id=1207

goodluck its a simple fix