Server randomly starts lagging.

Bukkit version 1317
Java version 1.7.0
x2 Intel xeon 5160s 3.0ghz
8GB ram
Windows server 2008 R2
Plugins:
Commandbook
Factions
Chestshop
Iconomy
Jobs
Mcmmo
Pvprward
Worldedit
PermissionsEX
Runecraft

No more than 12 users are on when this happens.

It doesn't always do this, but randomly the server will start spaming can't keep up messages and no one will beable to join, everyone seems to just be doing what the normaly do (not exploring new areas or anything) I can not find the cause of it, the console isn't spitting out any errors execept for can't keep up when this happens, I have to restart the server inorder to fix it. The cpu usage also jumps up to 30%