Large mailboxes

Roundcube seems to have some difficulty dealing with large mailboxes
(large meaning thousands of messages). It can manage indexing one of
these, but if I try to actually show a message in a large mailbox, it
basically hangs.

When this happens, the browser just sits there waiting for a reply,
and I can do a 'top' on the server and watch the apache process just
eat up memory until it finally dies.

It seems that Roundcube is digging through the mail and not releasing
memory in some way, which becomes a problem as the mailbox size gets
large. At least that's my theory.

Does anyone else observe this? This is with Apache 1.3.34, PHP
5.1.2, MySQL 4.1.18 and Cyrus 2.2.12, running on FreeBSD 4.11p14.

Thanks!

-- Mark Edwards