Ariston boiler microgenus 27 burner heats to max?

My Ariston microgenus 27 MFFI, gets into a mode where it turns on burner until all yellow leds are lit (80') before it turns burner off. At this point I can hear the water boiler inside. It does this even though the CH is off and no hot water is being drawn. Only way to stop it is turn the on/off dial to off. I can leave it off overnight and when switched on goes straight into 'burn' mode again. Its been doing this for a while, and I used to be able to get it back to normal by running the hot tap with boiler switched off, so cold water ran through the boiler. Any ideas where the problem is? (i'm thinking one of the two thermostats ...)