The server may cause client-side lag, because the builds might be active or server side lag and to much entities to keep up with. Try asking the owner or an admin if there's any server-side lag or problems with the host. Otherwise you might just have a bad computer or mods conflicting with the server script, if you do click force update, you won't have your mods anymore, but you'll be able to play on the server fine. Hope this helps!

Maybe the server is having difficulty keeping the server live,since 1.4 many servers are other trouble due to plugins not being updated,maybe ask some of your friends if they can join and play,if they can then I would suggest making a backup of your minecraft.jar and force updating (you may have some mods conflicting with the server script)
Please get back to me.