Boiler issue on boilermate ov a class system

I have a problem with the boiler which is an glowworm micron 80ff. We also have gledhill ov boilermate a class. Boiler intermittently has a non volitile lockout and did this over several months before fault code appeared suggesting a fan/aps problem and overheating. Had fan changed and worked fine and lockout began again. We now have error code again. If the boiler is left to cool it will work again but seems to constantly burn until it overheats and we get fault again. Any ideas of likely cause as it seems fan was not the problem.

Boiler pump.
Sludge as in partial blockage in flow / return pipe work boiler to unit.
Mike

Micron boiler has cast heat exchanger.
But very small in size and water ways.
Look at boiler pump,take head off and check impeller etc.
If unit is 3 floors or more up consider 15/60 pump too.
Mike