Vauxhall Vectra engine temperature @ 90degrees+ permanently

l asume in the morning when the engine has not run the gauge returns to zero? Then as you drive it finds its way up to 90. if thats so l have a question. does the temp stay at 90 even on a hot day when you are stuck in traffic? not getting hotter then cooling down when you get a clear strech of road? if that the case probably everything is alright... however if you are not happy,or its causing you to worry, i would change the thermostat. while that works being done, its a good time to back flush the radiator and refill with clean water and antifreez.