Electric showers heat cold water on demand through the integral heater tank.
If the water inside the heater tank becomes excessively hot, the TCO (thermal cut off) switch, a safety device is activated. This isolates the electrical current to the heater tank and allows the water to cool. Once the water has cooled to a safe temperature, the heater tanks electrical supply is restored and so the shower will again deliver heated water.
This issue may be caused by the temperature control dial settings or a restriction in the flow of water. If the temperature control setting is too high this can cause the water to overheat in the integral tank (please check the installation/user manual). The main stop cock or any isolation valves to the shower should be fully open so the water supply to the shower is not restricted.
Another factor which may cause the water to cycle between hot and cold is the build-up of limescale, dirt, shampoo etc on the showerhead. This build up can restrict the waterflow from the showerhead, preventing the flow escaping quickly enough through the showerhead, this means that water remains in the heater tank for longer than necessary, causing the water to overheat and then activating the TCO switch. The water will then cool and heat again and this 'cycling' could continue throughout the length of your shower.
To resolve the issue you should ensure that the shower head is descaled on regular basis, using a suitable descaling solution, particularly if you should live in a hard water area. It is also good practise to check the shower head an hoe will allow a sufficient flow rate for the electric shower that you have as well as check the shower hose for any kinks or blockages which may also restrict the water flow.