dimplex 2000 pumped shower?

The most common cause of this is a flow problem.
if the shower head is blocked with limescale or the hose is kinked, then flow rate is greatly reduced, this results in the water being overboiled (hot flow) and a safety cutout activates disconnecting the heating elements resulting in a cold flow which resets the safety cutout and the same cycle starts again.

try disconnecting the hose and shower head and checking if it maintains temperature. if not then there is a filter within the machine which needs cleaning. this applies to all pumped electric showers

Dave
May 2014