[WARNING] Can't keep up! Did the system time change, or is the server overloaded?

Can someone explain this? 3GB of RAM, server has 8 people on it, virtually no plugins.

 

This started happening to my server today.
I don't think it's my computer.
My build consists of:
i5 2500k
8Gb of DDR3
GTX560 SLI

I notice that these messages don't show up till I start using my browser (Firefox).
These messages are often accompanied by a lag spike in the server.

Edit: Turns out that this is a firefox issue. =S Did a fix and it made firefox much, much faster.
For anyone that wants to make their firefox faster, type about:config in the address bar and then search up browser.cache.memory.enable. Set this to false by double clicking.

 

Don't need to worry about it, my server has been doing it for 2 weeks and its running fine