IMAP slows down

[godovic]

Few days ago Zimbra server started to "freeze" for time to time, the users got IMAP timeout, and the web server is really slow, or the ppl can;t send mails especially with attachments.
So... what is the first point to start with to resolve this problem..

And how can I monitor - view all active connections ???
Maybe someone have a virus, and overflow server with connections.

The server is installed on Ubuntu x64 with split dns. 100 IMAP users

Zimbra and OS:
Release 7.0.1_GA_3105.UBUNTU10_64 UBUNTU10_64 FOSS edition.
Hardware:
Intel Quad Core 2.4Ghz | 8 GB DDR2 | 2 x 1TB WD RE4 RAID 0 for OPT