Unfortunately when your memory is running out there are only 3 options. One is to upgrade, the second is to reduce content and the third is to optomise scripts.
Although useful software (especially software such as Joomla!) does have a reputation and tendancy to start to like eating your memory. If these Joomla! sites have very little content you may find it easier to design pages yourself to replace Joomla as this would no doubt help with your memory issues.
Hope that helps
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