New shape Connect cooling issue...?

[Postman Pat]

I noticed the engine fan(s) running on after switch-off a few weeks back. The fans are now on for a good few minutes.
At the same time I've noticed while driving that the temp gauge jumps up to about 3/4 scale and the engine refuses to pull, maxing at about 59mph no matter how you drive. Then after some indeterminate period, almost like flicking a switch the gauge drops back to its usual 1/2 scale and the engine performs normally. No coolant is being lost, economy is the same although the coolant has lost its healthy orange colour and now looks like dishwater. Drove 270 miles on Friday with maybe 60% of the trip in this reduced-power state. I'm thinking not head gasket but maybe stuck thermostat or some fan control issue... thoughts from those who know this engine?

[get connected]

Has this been resolved?