FF 14 extra fast combi boiler?

My extrafast 96 will deliver DHW on a moderate flow but cuts out after few seconds on full flow from a single tap, displaying FF 14, i.e. over-heating, but re-sets again of it's own free will in a few seconds and runs hot providing tap is on moderate flow. First option faulty HeatingThermistor so I tried ex-changing CTN1 withCTN2 but that made no difference. Any ideas ?