You could have your rate set too high
Having he rate set too high can cause packet loss,meaning that you can get lag if you set it to 9999,though it doesn't always happen
For me I pay it safe and set it to 7500
Or it could be that your ISP is stuffing,meaning you can't do anythjing
It could be Valve doing something,like what happened before when we couldn't play online because WON auth servers went down
A large number of factors could play a role in your lag,you might also want to try finding local servers(which is almost impossible for me,because people in HK barely play ESF)
EDIT: *sigh I forgot about this site,which has a lot more tweaking variables you can find useful
http://half-life.gamehelp.com
Go to tweaks section,and you should be able to find more than 10 variables you can change to try and improve your connection to servers