There's a nice utility for monitoring the server, it's called zmstat-chart and you can find details on
this wiki page.
For instance, this command will produce a nice set of charts for a specific day:
Code:
zmstat-chart -s /opt/zimbra/zmstat/2008-06-04 -d /tmp/charts
Have a look at that utility and see what it shows about your server performance for tha past few days since your problem started.
I assume you are only running Zimbra on this server? if not, what else is running?