[Dovecot] Problem moving emails with attachments or HTML links

I recently switched from UW-IMAP to Dovecot (1.0.rc15 on Ubuntu) and am experiencing a weird problem that I haven't been able to figure out yet:

If I try to move a message from one IMAP folder to another, all works fine if the message is plain text. However, *sometimes* the message will not be transferred if it contains either an attachment and/or an HTML link (operation times-out). Not all the time, but just enough to be annoying. I've looked through the logs and for settings in the .conf file and can't find anything obvious that I've overlooked. Can anyone help me with any parameters that might be causing this type of problem? Thanks so much!

Wren Hunt Cambridge, MA