My Glow worm Ultimate 80FF hunts. ?

My boiler fires up as normal, it then shuts down and then fires up again, hunting all the time. I eased back the front cover and this helped, as once the boiler fired up as normal it stayed on. But now it hunts even with the cover off. Someone suggested that the seal on the cover needed replacing as it ought to be air tight, but this has not worked. Please advise. Thank you.