Bora cooling misery

[KPS]

w reg bora 1896cc se tdi pd, 91000 miles, main dealer serviced, owned from new.
Replaced cam belt; then later at intervals the water reservoir, the water pump and then thermostat. Problem now is at around 80mph the temp gauge suddenly goes hard right and STOP warning comes on. Just slowing the car down quickly brings temp down and I can drive on at 70mph with no problem. At end of trip I need to put about a pint of water in to top up reservoir. Car OK till next time I go over 70 on motorway (OK, ok I know one answer but the Bora likes doing 80!).
Garage now say it must be CH gasket - which is what I'll have to do I suppose; but 2 questions:
1. any other possibilities?
2. I can't get my head round what is going on if the CH gasket is the problem - why does it riun OK; then some event which causes temp gauge to go right up; then back to normal. What is the sequence of events in the engine to explain this. The garage view is that they can't explain therefore it must be CH problem. Any help appreciated. Thanks

[DeusLevitas]

the faster you go the more pressure is created in the cylinders.
which in your case at 80mph there is sufficient pressure to cause the head gasket to leak and cause overheating and/or pressurisation of the cooling system.
if your lucky the head will not be damaged.

[KPS]

To Deus Levitas
Many thanks. I had assumed that a leaky CHG would let water get to the outside: I hadn't considered that very hot gases etc might go from a cylinder into the water chambers in the engine. So I suppose the sequence is - leakage of very hot gases to the water system, causes the water to overheat/steam. temp gauge goes through the roof and water is lost through the water reservoir as it expands/steams? This makes sense now and I'll go ahead with the necessary work with an easier (if poorer) mind.
No-one mentioned hoses, radiator or radiator cap during the investigations and pressure tests and I also wondered why the CHG was suspect at high speed when these others weren't. Now I think I know. Thanks again. KPS