Potterton Profile cycles every 4-5 minutes?

My Potterton Profile 50e when started from cold works normally for about 15 minutes. Then it starts cycling 3 minutes on, 5 minutes off, 3 minutes on etc. even though the water has not reached temperature. Eventually, the water does get hot but it takes a very long time. It makes no difference if CH or HW or both are selected.

I drained the system and desludged it and also replaced the motorised valves but did not help. I read somewhere on the internet that it is probably due to a faulty flue fan but from what I can hear, it comes on normally and stays on until the bioler cycles off.

Any suggestions?