sounds like shower is overheating and thermal cut out is operating for your safety hence going cold and after a few minutes ok again ,check shower head is not blocked or partially blocked , if in hard water area heater in shower could be furred up inside causing hot spots could be replacement new heater element or shower im afraid . never take chances with bathroom electrics ,get a qualified competant person to fix it your safety is paramount to any financial cost.
ricardo
February 2009