Ignition experts needed

my son inlaw is having the same poblem with his 69 340 and we have not been able to find it his some time will run for upto a half houror just till the engine get warm then start doing the same as yours he does not have a tach and it is bacis stock with a mallory dual piont dist it ran well for 5 year with this set up. I am thinking that there may be a curtict beaker that is old and week that is causing this problem but where the hell is it.