prima 40 f boiler main burner repeatedly starts ans stops

I have a prima 40f boiler . I recently changed the PCB when it would not start up . Now when it starts the main burner runs for about 12 seconds , and stops , it then restarts after about 8 seconds ,and operates in this manner continuously .The hot water and heating are OK . Viewing the main burner flame , it appears to start "good" but then feathers away from the burner after about 7 or 8 seconds and then stops after 12 seconds . I can hear the gas solenoids operating correctly at start and stop . I am concerned that an underlying fault which is causing this start/stop ,damaged the PCB in the first place and dont want to damage another . I have checked the flu and it is corroded at the outer end , could this cause the start/stop symptom and if so how do i replace/ repair it.

1st check the gas pressure you may have a faulty meter governor

check the inner flue, probably rusted through.
a repair kit is available if this is the problem

When you say feather, do you mean the flame is lifting/wafting around.
If so turn off and call RGI, there is a problem with flue.

If the inner is leaking to the outer then thats potentially quite dangerous and could cause the boiler to explode!!!

Tony

Thanks to all for the helpl. Problem solved , the inner flue pipe is corroded at the end which exits the outside wall and has cracked and dropped into the outer . Repair kit to fit . Thanks again