Potterton / Profile Prima 40e boiler not fire up?

Allo:

My old conventional Potterton / Profile Prima 40e boiler does not always come on. It sounds like it has gas and water going in and but it just will not fire up. It sometimes will keep doing that for over an hour. It is happening more and more often. Also I can hear there are lot of bubble being pump through the central heating. I need to constantly have to bleed the radiators, at least once every 3-4 days.

I found out it would fire up when I keep switching it on/off and turning it up/down on the boiler stat and the room themostat. I usually keep the boiler stat below 5 (range from max,5,4...0) and the room thermostatic around 18-19 degree C.

dex
=:(