What Does this little Clock mean? game slowing down.

http://imgur.com/LpcC6Lj

We playing on a server. When i save the map to local its fine no lag at all, and the other lad on the server says he is fine too but the game is lagging for me and when it does this clock is flashing on and off all the time in the F5 menu next to the fps ups which are both a solid 60/60.

What is this problem and how can i stop it happaning?