Hot water cutting off on Baxi Boiler when running slow?

I have Baxi Bahama 100 combi boiler. When the hot water taps are run slow hot water comes on sometimes for a few seconds, sometimes a few minutes and then the boiler shuts down and runs cold, just before it shuts down it makes a banging noise, and shows flame failure . It has had a new hot water flow switch fitted but this has made no difference.
Can anyone help with what could be the problem,