vokera boiler?

when i open the hot tap the boiler fires up for about two minutes then cuts out, then fires up and cuts out then fires up and cuts out....... the diaphram which controls the gas valve had sieze on, which i fixed and also changed the thermister... checked the modulator coil for voltage ... which started at 9.75 volts and fell to zero untill the bioler cut out. the resistance at the modulator coil was 41.3 ohms. any ideas before i buy a new ignition control...