How can i fix an air lock in my heating system?

We had a new oil boiler fitted (Worcester greenstar heatslave 18/25)in jan this year (2009), occasionaly their is a loud bang which comes from a radiator, this isn't the same rad every time. The system was flushed twice before the new boiler was installed. It seems like an airlock because the bathroom cold tap when turned on runs then bangs and loses the pressure. if turned on slowly the water seems to run ok. How can i get rid of the airlock or do i get the installer back to flush the system again?