Our zimbra server is experiencing unexplained slowdowns every couple of days that are only resolved after rebooting the server. From the end user's perspective, if they are using webmail and they send a message, then it will eventually timeout. From the system terminal, there are slowdowns logging in, switching users, and restarting the zimbra services. It takes up to 2 minutes to change a user using 'su -'

Restarting all the zimbra services, dns services, does not resolve the problem. The problem is only resolved after completely rebooting. After rebooting, logging in, switching users, and restarting servers happen quickly.

We are using dnsmasq for split DNS which is needed for our environment because of NAT. But querying DNS returns results immediately.
We are using an external ldap database for authentication but no other servers using it show any problems and there are no load problems on it either.
Everything else is a default install and configuration.

There are no obvious errors in the system logs. The server load, disk IO, is the same when there is a problem and when there is no problem.

Originally this was happening once a week usually on mondays, or tuesday. This week, it happened on Monday, and Thursday.

My version is:

zimbra@servername ~ $ zmcontrol -v
Release 7.2.1_GA_2790.RHEL6_64_20120815212147 UNKNOWN_64 FOSS edition.

Has anyone encountered or solved such a problem ?

Thank you!