Sorry to ressurect this thread, but im loath to start a new thermostat one and this issue sounds like mine:
Start car and leave to idle
Temp hits 90 and stays.
Drive in stop-start traffic
Temp hits 90
Occasionally creeps up ( a smidge)
Fan kicks in
Back to 90
When on the open road the temp drops to 70, if I have the heater on it drops a midge below that as well. Doesnt move from 70.
I asked the garage to look at it a while a go and I think they just tested it by running it in the garage and seeing that it hit 90 and the fan kicked in. But it shouldnt sit at 70 on the motorway/clear traffic should it?