Air in system following upgrade

I have a big problem with air in the system. So far, four plumbers have failed to identify the cause, but each one visited once only and then "became unavailable". New oil boiler/cylinder/pump/controls were installed 6/7 months ago and the central heating and hot water is really good except for the new fault. The boiler has been given the all-clear (by manufacturer's approved plumbers). After 3 failed attempts to fix it, the installer has abandoned me and does not answer my messages. At random intervals we get a noise which ranges from a gentle tap to something sounding like air being forced under pressure into water. Then, always 20-25 seconds later, bubbles hit the pipes in the airing cupboard and this is accompanied by bangs and thumps loud enough to wake you from a deep sleep. Just once it appeared to be in the loft too, although I'm told air is not being sucked from the loft tank into the system. Whatever it is, must be happening in the pipes between boiler and cylinder I would guess. The boiler has made no unusual noises on the occasions I have been present during the episodes. Differences: * The old boiler was wall-mounted, the new is ground-based (in the garage). So presumably pump under a bit more pressure? * Controls are now wireless. Other info: * The now-absent installer was heard talking about a pipe which "wasn't really at the right angle". * During installation, he went through a noisy procedure of vibrating the pipes and radiators to aid sludge removal which I thought a little strange, as did one of the other plumbers! No sign of water leaking anywhere though. Unanswered questions from a non-plumber (me): * Could there be a problem with wireless signals and thick walls, and would that cause any related glitches (pump synchronisation etc)? * Is it possible that a vacuum is sucking air in through a dodgy joint in the pipes, leaving no evidence of a water leak?