how can i fix myson apollo?

my mson apollo boiler keepscutting out.I have recently replaced the thermocouple and limit stat.It has now gone off again,i can get pilot on burners fire up,it then makes akind of whining noise.Radiators and water begin to warm it runs for between 5 and 15 mintes then cuts out completely. someone i know who is doing aplumbing course at night school thinks the pilot and burners may need cleaning ,any further advice or help would be appreciated.