I've been experiencing the same problem on zcs-6.0.14_GA_2928_RHEL5_64 since I've upgraded from 6.0.13. Seems to be related to mailbox service on my server. The next time it happens I'll snag a ps output of the process and post it here.
Enabled services:
zimbraServiceEnabled: antivirus
zimbraServiceEnabled: antispam
zimbraServiceEnabled: logger
zimbraServiceEnabled: mailbox
zimbraServiceEnabled: mta
zimbraServiceEnabled: stats
zimbraServiceEnabled: snmp
zimbraServiceEnabled: ldap
zimbraServiceEnabled: spell
I only have top output of the process from today's event. 8G of ram on this server btw.
PID USER
PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7087 zimbra 18 0 4152m 2.4g 10m S 198.7 30.7 283:02.41 java
After a zmcontrol mailbox stop && zmcontrol mailbox start, load is around 0.5.