heater problem?

my ford Mondeo estate 2003 the temperature gauge shows normal when idling and the blows hot air but when you drive it the temperature gauge goes down to cold then back up to normal but then blows cold air it will only blow hot air when its idling again any ideas ?
dickie, December 2016


done
Solved it was the thermostat the two plastic clips were broken

Dickie, December 2016
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