Welcome to just Answer , There is a known fault as this and a repair to the engine coolant temperature sensor which includes replacing the sensor and its wiring plug with a much better gold plated terminal .
Your sympoms are exactly the same as the repair bulletin , there is an extract below with the part numbers that you will need to order from Vauxhall.
I hope this helps , best regards XXXXX XXXXX
Spare-parts: Part-No.: Catalog-No.:
Sleeve, cable, wire 90543865 12 86 663 connector
Hose, shrink, wire harness 90543875 12 86 666
Adapter kit, gold plated 93174557 62 86 420
Coolant temperature sensor 93174208 62 38 266 (Z14XE, Y16XE, Z16XE, Z16YNG)
Coolant temperature sensor 12992783 13 42 571 (X18XE-1, Z18XE, Z18XEL)