On my 2012 Countryman S I recently changed the water pump, replaced all coolant hoses (even the plastic one that runs behind the engine, thermostat, the metal/rubber hose on top of the turbo, and thermostat sensor. Only coolant related items not replaced was the aux coolant pump and the hose on it and the two hoses going into and out of the heater core. Put 50/50 blue coolant in, burped, and off I go. Heater puts out hot air, no codes thrown. Coolant heats up to 116C and 117C then rapidly decreases to 104C, then heats back up to 116C but slowly. Say a minute to heat up and 10 to 15 seconds to cool. The fan kicks in low speed at 108C and high speed at 112C. When I run the car in sport mode car runs normal temp, 100C to 106C with a much tighter range, say 102C to 105C. There is no lose of coolant. After I burped the system the coolant has stayed right were I filled it, the max line for over 100 miles). Prior to putting coolant in I drained all the old coolant (removed all hoses except for heater core hoses), put a vacuum on the reservoir and it held the vacuum. I used the vacuum to put the coolant in then ran and burped the system.

Any one have any ideas on what would be causing the temp flux in the coolant? I'm thinking maybe the new thermostat but it appears to be working right as the fan cuts on min and max at the right temps.

As info I did all this because I had to replace the clutch so I figured since nothing had been done to the car since I brought it I'd change out the water pump and hoses as they had 8 years on them and the water pump was the original and not replaced by mini during the recall because the recall was only if the water pump had failed.