Jim, If you have hard water that is almost likely the problem. Lime seperates from water at high temperatures. That why it is always your kettle and your shower or even central heating tanks. Ironically turning it up just makes the situation worse as more lime seperates . If you are a competant or qualified person one would isolate the relevant services and remove the service filter . Given what you have said this should show evidence of limescale. There is no cure. I would reccomend you have a water softening device fitted to protect your home. Then and only then buy a new shower. Mira make a Sport which has a device called clearscale. It inhibits the growth of limescale and is a robust and reliable product.
Look on the bright side hard water is very good for your teeth.
Alan, December 2008
