The shower runs very hot for a minute or so, then ice cold for about the same length of time, the cycle just continues.

Frank Horan, October 2014

There is insufficient water flowing so the water overheats so the safety cutout operates,after it has cooled down the cutout switches back on. So you need to find out why,try turning temperature dial more to cold, or there is sometimes a filter that can be cleaned dependent on the model shower

toptrish, October 2014