my new boiler is doing what my old boiler did-cutting out?

what could be the problem when you replace your boiler but get the exact same problems. we replaced our boiler 3 years ago and have had engineers out nearly every week and they dont know what else to do. our boiler cuts out after 15 mins and its getting more difficult to get it back on again. It used to cut out only once every couple of weeks but its now down to every 15 mins. We have had a powerflush, different bits replaced and 3 weeks ago the engineer fitted a red metal sphere type thing to our pipes - he barely closed th door and the heating went off. Im beside myself im not convinced its the boiler - is there air getting in? Do you have any advice on what might be causing this? Thanks