Halstead Ace High intermitant hot water?

I have an Aces high Halstead Boiler it is now six years old it has had a new heat exchanger plus the old descaled heat exchanger refitted twice, new diaphram & spring in the flow switch approximatley 18 months ago. The hot water is once again is palying up fluctuating from full heat to cold approximatley every minute whilst the central heating remains fine. It has been suggested that the thernistor's need replacing but the plumber who suggested this gave the impression this was the first step in the process of elimination possibley needing a new board next. I am some what wary of bolting on new bits and simply waiting to see if this solves the situation. Are his initial thoughts likely to be correct or should I once agian be looking to replace the diaphram washer in the flow switch?