If I take my 2001 MGF on a run it seems to reach running temperature rather quickly. Then, if I am sitting at the traffic lights, it starts to overheat and the cooling fan comes on. But... if I start it from cold, not intending to drive anywhere and just sit there, the gauge goes up to the right position (just below 'quarter-to-nine') and stays there - no overheating at all. The coolant does not seem to be dropping (I have marked the low-point and the high-point on the bottle), there is no bubbling or traces of oil in the coolant, no white deposit on the dipstick. I don't think it is the dreaded head gasket failure (HG was done 50,000 miles ago). What can it be? Air blockage? Water pump failure? Any ideas would be very welcome. 

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