Robert,
Good to see you back on the Forum.
Looks like some good advice above, but I just thought I'd give an opinion...
Your problem sounds very much like a radiator issue, to me.
If the temperature gauge capillary or bulb was broken, the gauge would not work at all.
I had a problem with my coupe, some time ago.
Would run fine all day if kept below 60mph. Go to 61 and it would start to boil, very quickly.
Turned out that the radiator was partly blocked with sludge and, over 60mph, the faster water flow through the parts of the radiator that were working would not allow enough cooling before the water flowed back to the engine.
Bryan bought the car, re-cored the rad straight away and has never had a problem since.
Good luck with the fix.
Let us know how you get on.
Robin.