Potterton profile 100s not firing?

Potterton profile 100s, really strange thing, boiler working fine then could not get it to fire until I turned the boiler stat to the zero position and then the fan started and the boiler fires. It has been running like this for an hour or so then cuts out, I press the reset and off we go again, just wondering if I am not getting a supply once the three way valve switches over on the internal contacts and am not getting full voltage at the boiler [ i have had a meter on this and the voltage is down]any ideas ?