Vaillant Turbomax 828 hotwater temp varying?

When using the hot water the boiler heats the water fine, but after 30 seconds the gas goes out, the water coold slightly then the gas re-ignites and the water heats - this cycle is then repeated, I also hear a humming sound coming from what seems like the heat exchanger - I recently drained the system. I wonder if air has got into the heat exchanger part of the system ?? it seems like the circuit boils the water - and it over heats - turning off the gas - is there a way to bleed this part of the system if air gets in ??