My T5 keeps over heating and blowing the coolant out of the header tank. When it overheats the fans in the car just blow cold air out even on the hottest temperature. When driving along there is some heat out the fans then it will get cold for a minute then get hot again. I had the head gasket done less than 12 months ago. Recently I have had a new coolant temp sensor, new thermostat and it has been in to the VW garage for the past week who cannot identify what the problem is! They have said it is maybe a blockage but not come up with where! Any help greatly appreciated.
From what you described it sounds like the coolant level is low or has air in the system. Of course if there is a leak internally or externally you will want to repair it right away. Keeping note as to how often the coolant level goes down is a good indicator of how big the leak is.
I have outlined some helpful diagnostics for an engine overheating situation.