I've got exactly the same problem. The trouble is that there is a moving part of the stabilising valve that reacts to water pressure and then pushes on a switch which is part of the control board. If this is a bit sticky, or if there is too much clearance, then the shower thinks that there is too little water pressure and shuts off the power to the heating elements.
The answer is to place a small shim (I used some stiff plastic about 1mm thick cut from a packet) between the valve and the shim, to take up the excess space. Obviously you need to ensure that the electric is off before you go anywhere near the inside of the shower
Jon, April 2009
