How can I fix my boiler? Water and heating doesn't always come on ?

My halstead quattro gold combi boiler sometimes works and sometimes won't come on for long periods. When ever we call out the plumber he thinks it's a few reasons but then it starts to work again so he can't tell where the problem is! Sometimes when the heating has come on it will go off on its own accord and it then won't come back on. Also when heating has come on sometimes goes off when hot water tap has been put on and heating won't come back on. Can anyone advise?