Normal operating boiler but sometimes doesn't provide hot water?

Hi i have an ideal c24 boiler less than 2 years old and the LED lights indicate it is a normal oeprating boiler, but three times now it has stopped giving out hot water when showering, any idea's what it could be, the thermistors just got replaced January this year, and its not stopped completely i have hot water one minute and nothing the next? Could living in a hard water area affect the build up of limescale on the boiler parts
Any info would be greatly appreciated
Thanks

Sam
March 2011