In that case i'm guessing it's your connection.
You've got several things you could do to troubbleshoot including
Joining another server to see if it's the same lag there (if it is then it's likely your connection)
Or when you'r experiencing the problems on the server then check your latency eigther by running
http://www.speedtest.net/ or trough cmd.
To check your latency with cmd then press your start menu, in the search field there write cmd and hit enter.
In the black window that appears write "ping xxx.xxx.xxx.xxx -n 25" where the x'es are the ip to the server you'r playing on.
Then look at the part that says time=xxms, if that goes above a few hundred then try to ping another site as well (for example google by writing "ping
www.google.com -n 25").
If your ping to the server is high as well as to the other site then it's likely your connection, if the ping to the server is high but to the other site low then it's likely the server.
Also, you'll probably get better answers in the server tech help section