DHW clicking on and off when in use?

Hope someone can assist......have a slight fault whereby when the DHW is being drawn, but not full (ie.shower) the hot water will click on and off. Showering is becoming an extreme sport now (boiling hot then freezing cold). I think it's something to do with the DHW temp sensor as when it clicks off, the fault light clicks on and then it's ok again. I replaced the diaphram a couple of months ago, so i don't think it's that. Any ideas......could it be as obvious as the temp sensor?

mjw2009
April 2009