worcester cd35i ii pressure problem?

i have a problem with my combi worcester cd35i ii the pressure of the unit is at just over 1 bar and when you either switch the hot water or the central heating on the relevant lights come on and the boiler kicks into action and the gas fires up, the pressure increases slowly to 1.5 bar then all of a sudden the pressure increases to just over 2bar and then the needle waves about and the boiler makes a banging noise as if there is an air lock and then switches reverting back to the standby mode and the pressure goes back to 1 bar, ive bled all the radiators, what else can i do or reset or change or is it an expensive plumber call out