main burner lights but goes off a few times before working smoothly?

Ideal Classic NF30. on first swith on Pilot lights then ignites main burner and burns as should for about 12 seconds but then main burner goes off, comes on again for about 6 sec, then off, then on again for about 15 sec. This happens about 8 times then all runs smoothly for hours. It happens everytime we switch it on and must be related to some part of system getting warm and functioning properly. boiler was fine until Gas Safe reg'd engineer serviced 10 days ago. Could it be to do with casing being replace incorrectly or dirt being put into components? engineer came back and said probably gas valve. I am sure it is not that but would appreciate diagnosis or pointers if anyone would advise.