Hot water cutting out?

Hi,

I have a vaillant 837E turbomax and from time to time the water will not heat on opening of a tap. When the system is reset via on/off it reverts back ok. However the other day I got an F22 code(dry fire protection) and again it went back ok when reset. I have tried bleeding all the rads, fine, the pump is blead and turning freely and the auto air valve is slightly open as expected to auto blead air from system. All valves are fully open and mains is too. I have thought about the divertor valve, but why if it resets ok?? Also thought about flow regulator as the hot water can sometimes go hot and cold on its own and when the draw from the tap is reduced it goes hot again. This leads me on to thinking of checking the cold water inlet filter to see if there is any restriction but don't know where it is, as this may be affecting the flow. Any clues to the filter location and problem generally???

Many thanks,

Graham Adamson