glow worm ultracom 18hxi low flame problem

[phil hair]

hi, Im looking at a glow worm ultracom 18hxi that heats up to mid 70s/80s works for about 20 mins, then goes into low gas mode (one flame on display) and stays there for the rest of time. no heat in the water or rads.
I have changed a 3 way valve (which was jammed with the black stuff)
I then changed the pump (whos impeller fell apart when took head off)
the pump then started sucking air into system so turned it to low setting, worked ok for a day, then problem started again.
they tell me that they had it flushed a few months ago and a magna clean fitted.
if i reset it a few times it starts working fine?
im thinking pcb now, any ideas?

 

[Gasmk1]

system could still be full of sludge