Fluctuating DHW on Ferroli Modena 102?

- I have tried all sorts to sort out this problem. As per other answers I tried switching the thermostat connections over and found that the hot water finally stayed hot. The advise then was that the thermistors were therefore faulty. I changed both, but it has made no difference. I can still only get the hot water to stay hot by switching the connections to thermostats over! Any fresh ideas would be very welcome,