Sounds like overheating due to low flow. Check the hose isn't kinked, and that the head spray plate isn't blocked with scale. Run the shower just on the hose, with the head unscrewed to check. If it still doeas it, check the supply pressure and flow rate are adequate. Check that any isolating valves are fully open, and that any inlet strainer is clear of debris and not scaled up. What area are you in? Local pressure depends on supplier and area. Are you on the ground floor of a single property, or do you share a supply (flat conversion) or live higher than the 2nd floor?
PlumbSpark, October 2009