My wife has an MGF that has developed a problem with the cooling water. All is well for most of the time, but when held up for any length of time, with the engine ticking over, the water temperature suddenly starts to rise and, if nothing is done, within a minute the water is boiling. If it is possible to continue at speed the temperature will fall back to normal, otherwise the only solution appears to be to turn off the engine when stopped, before the temperature has time to rise, and restart the engine when the traffic moves on. Do you have any idea what would cause this?

The problem has only started recently; the car is 10 years old and has done less than 40,000 miles. When this problem happens there is plenty of coolant/water in the system. The garage suggested it might be a dodgy thermostat, but fitting a new one did not cure the problem. I wonder if you have met the problem before and could guide us to a solution (preferably one that will not cost more than the value of the car)?