in heating mode burner comes on and after short period of time goes out.After a short period it comes back on and then goes out again.With a tap running,burner lights up and goes out after short period of time.

Stewart Soppitt, May 2017

You have a combi boiler?

Burner doesnt stay on for heating & hot water?

You can hear a fizzling noise from the boiler when on?
Like a kettle boiling?

You have sufficient water pressure?
Indicated by water pressure gauge on boiler

All radiators have been fully bled/vented?

FLOW & RETURN temperature differentials within manufacturers specifications?

Burner gas pressure within manufacturers specifications?

Pump operating correctly?
Not seized

From the limited information you have provided
I would say the boiler is overheating & cutting out on the
High limit temperature stat

Why is the boiler overheating?

Would need to do additional tests to determine cause eg
Temperature differentials
Burner pressure
Gas rate
Flue gas analysis
System water quality analysis

Derren brown, May 2017