The Golf Mk5 was introduced in Europe in the autumn of 2003, reaching the UK market in early 2004. In North America, Volkswagen brought back the Rabbit nameplate when it introduced the vehicle in 2006.
I can't say for sure what the problem is, seeing as how I haven't inspected your vehicle in-person. I would guess that there is an issue with your radiator, and you may want to take a look at it.
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