myson boiler?

I have aked aquestion earlier on about my mson apollo boiler, ihave problems with it lately i have resolved acouple of them lately 1stly by replacing thermocouple and had afurther issue ,this was resolved by replacing limit stat.Recently it has failed again , iget the pilot to reignite burners fire up but after about 5 to 15 minutes it would fail again.However ileft it off for a few days have just put it on again it has ran for about 35 minutes radiators and water are waming. I have just switched it to the off position and noticed the pilot light is ayellow colour ,could this be causing the problem, any ideas or help please.