myson apollo 40si keeps cutting out, I thought it maybe the air pressure sw?

Myson apollo 40si keeps cutting out. I thought it was the air pressure switch as unable to get it started. The hose was found perished and replaced it. Boiler started up. left running, then I switched it on and off several times all was okay. However cut out again. ANy ideas, is it the thermocouple or the over run thermostat or gas valve