Thanks, Alex

Still seems to be somewhat of an issue, though. Sort of perplexing:
Accessing
http://www.jfsyork.org/silverstripe/?flush=1does NOT cause an error, whereas accessing
http://www.stayingconnectedyork.org/?flush=1DOES cause an error. Very strange! They're running the exact same software with the same configuration settings. I removed all unnecessary files from the stayingconnectedyork.org Silverstripe install, and in fact the jfsyork.org/silverstripe install has a lot more template files and page types, that could potentially increase the memory usage when the cache is flushed.
Any thoughts?
Edit: I should add that I was meeting with the client today, and we encountered the error several times without explicitly calling a cache flush using ?flush=1, so it still remains a problem for general users of the site.