Glow-Worm Space Saver problem?

I have a 20 year old Glow Worm Space Saver 38 Boiler which runs fine most of the time but has started to occasionally suffer
with what sounds like the on/ off valve pulsing open and shut
(bangs while this is happening) - the flames pulse larger and smaller while this is going on. Only happens during heating
operation (not when heating water only). Can go for days without happening and then all of a sudden can start again.
Boiler has just been serviced - but happened before and after
service - would be grateful if any body has any ideas.