Coolant Temperature Issue

I was getting a CTS High Input Fault and the car was idling poorly and sometimes cutting out.

I changed the CTS and started with opcom plugged in. I noticed the coolant temperature was fluctuating randomly and when it read -40 degC the car would cut out. I unplugged the CTS and plugged back in with the car still running and the temperature started to fluctuate again but then settled at the correct temperature.

I seem to have this issue on first start only. Any ideas what could be causing it?