Ideal boiler code FU intermittent..

[Les Murkitt]

I have a 4 years old Ideal boiler not a combi but a indirect heating boiler not sure if the model is a "Logic" or what. Anyway after being away for the Winter Nov - May boiler ran well then after a couple of weeks we had a clanging noise after the heating cut off & the boiler screen showed the temperature going up dramatically to around 94 degrees it took 30 minutes or so the come back down in the meantime the radiators were just slightly warm. Following that all was well until 2 days ago when it did excactl;y the same & showed FU code on the screen. FU appears to mean a blockage within flow & returns possibly in the boiler itself but if that is the case how can it be intermittent? Any information on anyone experiencing similar problems I would gratefully recieve. Thank you in anitcipation...