What's causing the problems with my temperature gauge in my 2001 Ford Mondeo?

The temperature gauge oscillates between optimal and maximum on my 2001 Ford Mondeo 1.8 petrol. My heater doesn't work well either. Most of the time it blows cold air and occasionally gives a burst of hot air. I changed the thermostat, but these problems are still there. Any ideas what it could be?
Probably sludge in the cooling system of the car and the heater matrix. Try Radflush.
Answered by Honest John

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