intermittant turning off heat?

Hi , Vaillant vcw gb 242 eh - I know it's old . This system turns off for no apparent reason . Even when it is on the 2 radiators in living room are not as hot as the others . I can get it to work again if I leave it off completely for an hour or so then turn it back on again but it can turn off again at any time . Under my british gas contract I have had 9 different engineers who can't fix it . any geniuses out there ? Ta !