Valiant vuw 242/1e dhw fault?

Have a client with above boiler who's dhw would only work when there was a large demand-eg tap belting out. Got the diverter valve service kit and changed all the bits. Have adjustd the microswitch back and forth soo can get the boiler to fire with what ever flow you like, only it cuts out on overheat when the flow drops- filling the bath is ok, but no good for shower. Central heating is appears to be ok.