pro combi 85 hot water fires up on and off every 20-30 seconds

[julesverne]

hope someone can help on this as im stumped.
heating works ok with no problems.
turn hot water on it get hot then burner goes out then doesnt fire up again till water is stone cold then fires up again. temp gauge doesnt come back on till it reaches around 2c

dhw thermistor seems ok steady reading .wondering if plate hex