Intermittent Baxi combi fault

[cheshireblue12]

Any advice gratefully accepted daughters Baxi combi fires up ok but then locks itself off without getting central heating hot. Does this a number of times then eventually stays on and keeps central heating warm. It appears that central heating water temperature has no control at the boiler itself. I have witness boiler producing hot central heating and showing no water temperature(below bottom scale) then temperature indicator can fly up to >80 deg C without any noticeable change in water temperature and the cycle goes on.
Any ideas please?

 

[bazdaman]

Primary thermistor issue I would say