Jumping temperature
 

Hi. recently whilst in traffic my temperature guage rose alarmingly and at the same time the heating blower went cold. As I got moving again the temperature went down and the blower returned to normal. There is coolant and I know the fan is working as it came on when I parked the car up. Has anyboby any ideas what might be the cause. many thanks

My first thought was to be sure there was sufficient coolant level in the reservoir; if it's below the "MIN" mark, the heater will blow cold air. Sounds like you've checked that, though.

My next thought would be the thermostat sticking or a coolant flow line getting blocked somehow...?

Stuck thermostat, faulty water pump, air trapped in cooling system. I'd start in that order.

We had a weird overheating issue with our 2006 Cooper S. Still under warranty and they replaced the thermostat, the fan and a sensor of some sort. Don't recall exactly what parts but the problem was that the fan wasn't going on when the car was stopped or crawling in traffic.