How to fix a Potterton Netaheat 10/16 Electronic boiler?

The boiler fires ok and the main burner ignites however every 20/30 secs you hear it try and ignite again. when it does this you see the main burner extinguish for a fraction of a second. ie the flames drop down and immediately come back.
I replaced the Electronic control unit, Spark Earthing bracket, Electrode and elkectrode lead. still the same.
I suspect the gas valve.
Any ideas would be welcome.