Aqualisa Quartz Shower Suddenly Running Cold?

I have just installed a brand new Aqualisa Quartz 9.5 shower unit to replace one that failed. The unit works fine until I turn the water temperature control knob to about the 1 o'clock position.

Running it at this temperature is OK for awhile but every so often the water goes ice cold for about 20 seconds and then recovers to hot again.

Is there a temperature sensor in this shower that I could adjust to prevent the heater cutting out?

Thanks for any help!