A few months back my 2004 Ford Focus 1.8 developed a chronic misfire, which was diagnosed by my Ford dealer as a blown 'coil pack' caused by 'unserviceable' spark plugs. Shortly after that was fixed the temperature gauge suddenly went up, almost into the red, while driving on a hot day in slow moving traffic. There were no leaks, no noises, and the gauge dropped again when I slowed down. It was a pattern that repeated over the next few weeks. The dealer diagnosed a blocked radiator so I had a new one fitted, along with a new water pump, but then it started doing exactly the same thing again. Suggested remedies range from faulty senders or ECUs to blown head gaskets, and the most likely seemed to be a burnt out 'slow speed fan resistor'. I took it to an auto electrician, but he couldn't find a thing wrong with it. I'm mystified, what could be wrong?
I had a similar situation on another car, not a Focus. There was a problem with the temperature sensor that switched the cooling fan on and off. I would watch the temperature gauge rise to the point it was almost at boiling point when I was stuck in slow moving or stationary traffic, but it would drop again when I was able to get moving and pick up even a little speed. The problem was that the fan wasn't kicking in to keep the engine cool when it was needed, but the airflow was enough once the car was moving to do the job. The answer in my case was to run with the air-conditioning on all the time, because that way the fan was always running. I have never had the problem again. Mind you there is still a problem with the sensor, all I have done is circumvent the issue. I would suggest you try driving with the air-con switched on and see if the problem persists. If it doesn't you can be sure it's a problem with the temperature sensor that controls the cooling fan, if it does persist it could be the fan itself.