Overheat

[Skullb]

I am being driven crazy by this . Last August When I put my 2001 986 Boxster auto in for its mot the local garage informed me it was overheating - they said to leave it with them - It got its mot and after driving about 4 miles from the MOT station to get the road tax the temp gauge started to rise and by the time I got back to the garage about 1 mile later it was redlining.. when they removed the coolant tank top it was like a hot spring shooting into the air.
Left it with the garage . They inform me cylinder head gasket must be blown . Will cost up to £3500 to fix but can’t be sure till they start stripping ???
As I couldn’t afford the outlay they took it back home.( not where I live ) since then I have taken the car out on several occasions to see if the problem can be replicated . Today I drove the car to get up to operating temp and then about 20 miles varying between 55 and 75 mph . The temp gauge stayed static at operating temp. Cooling fans were working. Air con on cool ..... ???? Anyone got any ideas ? - could the overheating have been merely a rogue happening , dirt in thermostat etc etc why is the car still not overheating ?