i have the problem with my ferfoli DOMIcompact F30 B, the hot water keep?

i have the problem with my ferfoli DOMIcompact F30 B, the hot water keeps going hot and cold.
i have changed the dhw sensor but has only lasted 6months.
is there any way of checking the sensor before fitting as i have just replaced with a new dhw sensor and i have still the same problem should i use some heat sink paste on the sensor???????