on the AE-list, Michael Natkin's advice on AE7 out of memory problems:
'You can generally help these problems by going to Preferences > Memory and Cache and *lowering* the value of Maximum RAM Cache Size. You are probably running into a situation where memory is fragmented, just like can happen to your hard drive. Try knocking it down 5-10% at a time til the problem goes away. (And yes, we are aware this is not really good :).'