This is a pure Plumbing problem and Nothing at all to do with your AGA, the Aga only heats the water!
It sounds like you have a high point in you hot water distribution system, which may be being assisted by a poor cold water supply to the storage tank, which is running dry when using a lot of Hot water, (Bath/Shower etc) leading to an air lock.
Please don't blame your oil Aga, these get enough critism with carboning burners without being blamed for anything else!!
www.elanar.co.uk, August 2009