Having terrible lag on a certain server
Solution 1:
There are several things that could be causing this:
Proxys, if you're using a proxy it could delay some information and not others.
Magic Launcher, I have found that having Magic Launcher's console open will stop all incoming connections when it receives a Coloured message. this can be fixed by launching it without the console.
Distance, if the server you play on is physically further away, then it could severly delay information going between you and the server, especially if you have a slow internet connection, to find where the server is located go to http://geoip.flagfox.net/, type the server IP or domain name in the field that looks like this:
Server lag, if the server's TPS(Ticks Per Second) is reduced, then it will start giving up on those with a slower connection. this can be fixed by getting a better internet provider.