Temp rising when idle ?

Hi everyone i have an issue thats got me stumped so hoping someone here can help. I replaced the head gasket on my rover 25 1.4 si last week as the original one was failing. Had the head skimmed and used a MLS replacement. all went back together fine and runs now without loosing coolant. BUT when i am stopped in traffic the temp gauge rises quite quickly upto almost the red line. (It might even get there but i always stop the engine before then) but when i start moving again the gauge drops to its normal position just as quickly. The fan comes on as it should. no air comes from the bleed value when i check. plenty of coolant in the expansion tank. so what could it be. could it just be that the thermostat is giving off a false reading or is there something im missing? any help would be greatly recieved!

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