fix Worcester 350 combi boiler ?

I have a Worcester 350 combi. Heating operates fine. Hot water runs fine except for the shower. Think reduced flow causes boiler to cut out. Boiler cuts in and out during shower operation. It seems to be just on the margin of operation. Operates if shower temp increased but then too hot to use. Used to work OK. Have changed HW thermostat. Have bored out shower head to increase flow which worked for a while???