We have exactly the same problem,the pressure dies off as the water reaches the right temperature. We have had the thermostat replaced and it still hasn't made any difference. Lowering the shower head to floor level increases the flow. The shower is a Trevi Therm Mixer Shower 2002 model
I would suggest that you have a pump fitted. It seems to me your hot water cylinder is either too low to give a good gravity fed pressure or your cold water storage tank is not high enough above the cylinder. Lowering the shower head to get more pressure is a good indication that the pressure required cannot be supplied by gravity alone
Keith
September 2007
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