problems with worcester 350 hot water supply?

Can anyone help me fix a problem with my worcester 350 boiler? It runs ok on heating mode but when a demand for hot water occurs it runs until hot water appears at the tap but then locks out requiring a reset of the stat. on top of the case. Hot water pump is not seized ,diverter valve seems to work as it is the same whether the boiler has been running hot or from a cold start. hot water sensor seems within limits of spec when tested with a meter. any ideas where I look next ?