Glow worm Energy saver?

Hi,
My Glow worm Energysaver boiler starts to resonate when Full hot water is demanded, it sounds like a noisy car on tick over and there is a slight smell of gas. It's not water hammer, it's combustion related, maybe the gas valve is defunct. Once it's power cycled it runs ok providing you don't open the hot tap to full. Central heating is working ok.
Thanks,

Any suggestions?