My boiler's gas burner tube is catching fire, how do I fix this?
For most of these possibilities you will need a professional service company to fix. Some possibilities are; 


The burner tube may be worn out, distorted , or dirty. 
The delivered gas pressure may be too low. 
The chimney draft may not be correct for this unit. 
Is there any thing stored near the boiler that could give off an odor or vapor that could influence the gas mixture. 
The burner orfice may be worn out or partially plugged. If the boiler is an older model this could be the problem and the orfices need replaced. Or, 
the orfices may need to be removed and cleaned.