Mine is doing sort of the same, but it wobbles and sometimes goes up high, but if I'm on the motorway for a bit like a minute or two at 80 km/h it drops to the bottom super fast, and although the fan never kicks when it goes a bit highee than mid, after inspecting under the bonnet everything is good, and coolant is almost at the lid of the radiator and not boiling or anything :s I'm at a loss 
