Garage "sniff" test - a gadget that basically shows if there's exhaust gasses in the coolant ...........................
 
Thanks reiny.

The thing is, it doesn't happen all of the time. If it were a HG failure, wouldn't it be constant?

I would be inclined to say yes but these things have a mind of their own. A few months ago, a friend's 300tdi was displaying the same symptoms yours was. We took the head off, took it for a skim, some spots needed filling, we got it back looking newer than new, re-fitted it and it got worse. The head was cracked internally.
 

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