How can i fix my central heating system?

can anyone help I have a fully pumped ch system which has started to play up i.e. after running for several minutes the vent pipe starts banging and crashing and i have to shut the system down. I have bleed everything several times and it doesnt seem to make a difference also it doesnt matter whether i run the pump on one, two or three. There is an air seperator above the cylinder could it be blocked? Any ideas appreciated. Thanks