Triton Bewitch Electric Shower 9'5kw, Constantly going hot to cold

[mack625]

I have a Triton Bewitch Electric 9.5kw shower, which keeps going from hot to cold every 20 seconds or so, it has three heat settings buttons with led lights on the front panel, when it goes from hot to cold you hear a click noise and the led lights show it's going from hot then to cold, the water flow rate is fine, i removed the shower head to make sure no build up or blockage there, i've tested the thermal cut out with a meter and by putting the live wire to the other side of thermal switch but still has the same problem, i've turned temp down to minimum but still without any success, as the flow of water is fine and thermal switch is not faulty has anyone got any ideas what else would cause this problem i could test.

Thanks