I know that there are some plumbers on here, I was just wondering if you could help?

It is a problem i am having with the Boiler , Baxi 105(combi)

When i am running hot water for a bath after about 30 seconds the water starts to run cold for about 20 seconds then warms up again , its as though the boiler keeps cutting off and letting the water go cold .

It seems ok when you are having a shower and the central heating works (i did notice theat the little LED above the flame symbal took a while to come on when i first started the central heating)

thanks for any help