My car’s (2015 1.6 CR TDI 182k miles) temperature gauge is all over the place. Flat at the bottom on the motorway and goes up to 80 doing short runs (it’s a taxi) then drops back as soon as you hit the dual carriageway or motorway. There’s still heat coming into the cab when it’s flat at the bottom but when it’s up near 90 there’s a noticeable drop In performance. It feels like it’s struggling a bit. 
 

I’m hoping it’s just the CTS as I have a replacement here and will get it fitted but I’m a bit worried it might be the thermostat due to the location of it. 

Thought I'd close off this topic with the resolution.

 

Changed the coolant temp sensor and it made no difference. Had the thermostat changed today and it's fixed thank God. Car feels better to drive too although that may be my imagination.