Hot water turns cold then hot

Hi, i recently moved into a flat and have a Pulsacoil A class boiler. the problem i am having is when i turn on any of the taps cycles from hot to warm to cold and then back to hot again and repeats the same cycle. The system appears to be working fine, what could be the problem

Thanks

The temperature sensor and /or pump require replacing, contact a local engineer from the list on the furum.