My Vokera compact keeps cutting out. What's wrong?

This mainly happens when hot water tap is on as it runs warm and then freezing and we need to reset the boiler. It did happen also when we were on holiday. We came back and house was freezing. Heating was on timer but had tripped out. Prior to this it had only ever happened when a hot water tap was on. British Gas been trying to fix this for past 2 years and have replaced just about everything but still problem is there. Any advice as I am at my wits end with this. It's now going about 4 times a day and is a pain in the but when happening whilst bathing kids as heating in garage and need to take kids out of bath to go and reset.

Angela, South Lanarkshire
May 2008