riello rdb burner only coming on for 5 min?

hi.
my riello rdb burner was airlocked and I bled it. It restarted and since then its only coming on for 5 min switching off and turning back on a few min later. It continued this for about an hour this afternoon. I have now switched off the whole system. It normally burns for at least 30 min and then switches off. I had the heating on a 1hr booster so the system was looking for continuous heat.
firstly is this switching on/off cycle damaging to the burner?
secondly any ideas how to fix it?
Tomorrow is Paddys day so it'll be fri before I can call a plumber.