Boiler shutting off during heating hot water

[Dumbaskus]

I have a Viessman Vitodens combi boiler. When running the hot tap (or shower), the water comes through hot but then the boiler shuts off, water gets cool, boiler fires up, water gets hot, and so on... Only started happening a few weeks ago.
I have replaced the flow sensor, checked voltage where the sensor plugs in to board and voltage varies with flow, so I don't think it's it. I have taken the thermocouple out from the top of the boiler and given it a good clean (was a bit gunked up).
What else could it be?

I'm not a plumber/engineer, just a DIY'er...

TIA

 

[ShaunCorbs]

Time to call a pro out or viessmann if it’s under warranty

 

[Dumbaskus]

Time to call a pro out or viessmann if it’s under warranty

Thanks, have also contacted Viessmann. It's at least 10 years old.