I agree with everyone else's solution of trying wired, but I am going to go a little further in-depth on the situation. Try restarting your router on occasion to allow for it to have a "fresh" start, as you would do with most electronics such as phones and computers when they start running slow. Also, if you have a router than you can configure the ports on, i.e. a gaming router(mostly because that is what I prefer), then set up rules allowing for more ports to be open for CS:S, that should maximize the send and receive rate of packets. You really want to run wired everytime you play a game online, especially multiplayer games that combat is involved to ensure the best results and response time.

I went through this link and it seems they broke it down pretty good for instructions, I'm sure there are better ones with more detail but I assume you're not a computer illiterate so I don't think Barney style is necessary. http://css.gamebanana.com/tuts/1673