Server Lagg issues - Need Help

I have in a month search around for errors that could my server to lag but can not even find a solution to it.
So if there are some in here who can help me I would be so happy.
Plz help out
I can be contact here or on Mail: th@itfree.dk or skype: mccocaina

Console error:
[Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 13777ms behind, skipping 275 tick(s)
[Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 14288ms behind, skipping 285 tick(s)
[Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2165ms behind, skipping 43 tick(s)
[Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 14322ms behind, skipping 286 tick(s)
[Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 13923ms behind, skipping 278 tick(s)

Normal player online 30-40
CPU on 10-15%
Ram on 20-35%

Linux vertion:
Distributor ID: Ubuntu
Description: Ubuntu 14.04 LTS
Release: 14.04
Codename: trusty
Linux 3.10.23-xxxx-std-ipv6-64 x86_64
Linux version 3.10.23-xxxx-std-ipv6-64 (kernel@kernel.ovh.net) (gcc version 4.7.2 (Debian 4.7.2-5) )

Java:
java version "1.7.0_05"
Java(TM) SE Runtime Environment (build 1.7.0_05-b06)
Java HotSpot(TM) 64-Bit Server VM (build 23.1-b03, mixed mode)

Spec:
CPU: Intel Xeon E5-1620v2 4c/8t 3,7 GHz+/3,9 GHz+ http://ark.intel.com/products/75779/Int ... e-3_70-GHz
RAM: 64 GB DDR3 ECC 1600MHz
Network: 1 Gbps

Plugins:
WorldBorder, Orebfuscator, LinksOnSigns, GroupManager, WorldEdit, ColoredBroadcast, ServerMotd, BukkitCompat, Buycraft, NightVisionGoggles, mcore, Vault, CombatTag, NoobResponse, WorldGuard, HeadDrops, DamageIndicatorHolograms, Lottery, LongerDay, WhatIsIt, ChestShop, Autocrafter, ClearLag, SpamExclude, Votifier, mcMMO, Essentials, floAuction, SimpleRegionMarket, SimpleVoteListener, EssentialsProtect, EssentialsSpawn, EssentialsAntiBuild, DropParty, CompatNoCheatPlus, EssentialsChat, Factions, NoCheatPlus, ScoreboardStats, FactionsPlus
http://dev.bukkit.org/bukkit-plugins/worldborder/
http://dev.bukkit.org/bukkit-plugins/vault/
http://dev.bukkit.org/bukkit-plugins/votifier/
http://dev.bukkit.org/bukkit-plugins/si ... -listener/
http://dev.bukkit.org/bukkit-plugins/si ... on-market/
http://dev.bukkit.org/bukkit-plugins/servermotd/
http://dev.bukkit.org/bukkit-plugins/scoreboardstats/
http://dev.bukkit.org/bukkit-plugins/orebfuscator/
http://dev.bukkit.org/bukkit-plugins/nocheatplus/
http://dev.bukkit.org/bukkit-plugins/ni ... ongoggles/
http://dev.bukkit.org/bukkit-plugins/mcore/
http://dev.bukkit.org/bukkit-plugins/mcmmo/
http://dev.bukkit.org/bukkit-plugins/longer-day/
http://dev.bukkit.org/bukkit-plugins/linksonsigns/
http://dev.bukkit.org/bukkit-plugins/autocrafter/
http://dev.bukkit.org/bukkit-plugins/buycraft/
http://dev.bukkit.org/bukkit-plugins/clearlagg/
http://dev.bukkit.org/bukkit-plugins/co ... plus-cncp/
http://dev.bukkit.org/bukkit-plugins/colored-broadcast/
http://dev.bukkit.org/bukkit-plugins/combat-tag/
http://dev.bukkit.org/bukkit-plugins/da ... holograms/
http://dev.bukkit.org/bukkit-plugins/dropparty/
http://dev.bukkit.org/bukkit-plugins/essentials/
http://dev.bukkit.org/bukkit-plugins/floauction/
http://dev.bukkit.org/bukkit-plugins/head-drops/
http://dev.bukkit.org/bukkit-plugins/lottery/
http://dev.bukkit.org/bukkit-plugins/noobresponse/
http://dev.bukkit.org/bukkit-plugins/spamexclude/
http://dev.bukkit.org/bukkit-plugins/worldedit/
http://dev.bukkit.org/bukkit-plugins/worldguard/