Unable to control the water temperature?

I have a 9kw sports shower which is usually on the 'High' setting. Recently the 'HOT/COLD' control has become very sensitive to adjust the water temperature, as one moment the water is very hot and by a miniscule adjustment towards a cooler setting, the water suddenly goes cold and the flow light extinguishes. I've checked the inlet filter. One would have thought the Flow light would have stayed on if the adjustment allowed more water into the system!