The Zimbra web client often hangs while typing messages. It is happening to all users for both IE and Firefox. The hangs can be brief, causing words to have typos of missing letters, and sometimes the hang is so long that it gives error "cannot connect to server". The problem seems to have first appeared after upgrading from version 6 to 7 several weeks ago. Usually a server reboot fixes the problem but not this time. There is plenty of disk space (over 100GB), only half the total memory of 4GB is being used, and both the OS and Zimbra are up to date. Suggestions? If a log should be checked, exactly which one(s)? Thanks!

NE 7.1.2 GA Release
Red Hat Enterprise Linux 4 64-bit