Halstead ace high burner problem?

I have a Halstead ace high combi boiler. I have replaced the diaphrams in the flow switch CH & DHW and removed and cleaned the switch itself from the main body of the groupset. I have also replaced the thermistor on the DHW side because it was faulty. My problem is now the DHW works fine but the burner only stays alight for about 30secs then goes off for a while and comes back on for 30 secs and repeats when i set the CH to come on.. any suggestions please. i suspect the electrodes in the burner but help would be much appreciated.