Sounds more like the gas to your thermocouple gets robbed when the main burner comes on. Thus reducing the heat, burner goes out, heat builds again on the thermocouple, boiler lights:etc.
Check thermocouple for soot or debris, clean gently, check nut is pinched up on valve end of thermo: Then check gas supply to thermo isn't blocked slightly(doesn't take a lot) Should sort you out.
Steve Lang, December 2009
