Players randomly cant hit mobs/place blocks

My players have sometimes been telling me that they sometimes (far from always) cannot hit mobs or place blocks. I once saw a person fight a zombie horde, his arm swinging all over, but the zombies did not react.
Has anyone tried something alike, and does anyone know a fix?

 

That sounds like lag. Can you provide us some additional informations? Server specs, OS, plugins, etc.?

 

It does not lag.
I have a 2gbps connection, 2 gb ram, 3 GHz core for the server in all. Im pretty sure its ubuntu 12.04

Also, none of my players have ever experienced chat lag or anything alike. Mobs moved without lag, they just couldnt hit them.
Dont come running up to me saying its lag. I know what i am doing.

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

2GB RAM and 3GHz doesn't mean it can't be lag! Like i said already before: Can you provide us some additional informations? Server specs, OS, plugins, etc.?

 

I just gave you my server specs didnt i?
Ubuntu server 12.04
2gb ddr3 ram
3GHz processing power (XEON)
2 gb p/s
SSD

And world guard.

 

Disable NoLagg and try if you still have this problem.

 

Happened before nolagg.
I installed nolagg a couple of hours ago. This has been going on for almost a week now.

bump. Happening again

EDIT by Moderator: merged posts, please use the edit button instead of double posting.

 

Then try to remove NCP and report back.