Alpha 240e DHW cutting out?

[Taipan]

Hi Al;

I have a Alpha 240e combi in a bungalow. When I run the hot water into the sink next to the boiler it behaves fine. But if I try to run a bath in the bathroom about 15 feet away the boiler keeps going out then reigniting and trying again. We have to do get a hot bath out of it but have to run it fairly slowly and just leave it to kick in and out?

I dropped the front panel of the boiler an watched it and the divertor valve seems to go in and out instantly but thats about as much as I know. Any ideas on what this could be?

Many thanks