I do not have a solution, but some info on what the problem is.
It is specific to your (and my) server environment, typically a shared host with lots of stuff going on, and a bug in the imap php module...
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=478193(http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=478193)
http://www.webhostingtalk.com/showthread.php?t=710876(http://www.webhostingtalk.com/showthread.php?t=710876)
These posts describe the problem exactly, but I have yet to find a solution or even a work-around. Please can someone maybe offer an alternative to the php_imap extension that would avoid this problem... and how such a replacement could be implimented on a shared linux apache server?