First of all, a high ping doesn't always mean your internet sucks.
There are several things that can cause your lag, I'll sum a few of those.
1. Wireless internetEven a closed door can cause lag spikes. Although this probably doesn't
cause your problems, I would check for obstacles that might be
in the way of your wireless connection. If you don't have wireless, then
skip this step obviously.
SOLUTION: Check for obstacles
2. ProcessesThis is what I think will cause your lag. Try to check the processes your
PC is running. A virus scanner might start running while in-game, because
scanners often think you are not using the PC while the game is running.
SOLUTION: Run Gamebooster to close everything you don't need.
=>
http://majorgeeks.com/downloadget.php?id=5952&file=15&evp=dc8312b3c3f52e18c69b7584026d7aec(Note: only install gamebooster, no other software :p)
3. UpdatesSome updates of your hardware actualy exist to help you. Try to check if your
soundcard, graphcard, ... are up to date.
SOLUTION: check the sites of your hardware or use Gamebooster to help you.
4. RatesAlthough you've said your rates are fine, I would double check it.
SOLUTION:
rate 25000 (or 30000/35000)
cl_cmdrate 66
cl_updaterate 66
5. DefragI don't know if you have problems with any other online games. If so, then
it'll probably be your PC. If not, there'll probably be something corrupt about
your CS:S.
SOLUTION:
Try to defrag your Counter Strike Source. You can do this with Steam, but also
with Gamebooster. I suggest you trust Steam on this.
If the problem still exist, I would simply clean your PC and run a complete
Hard Drive defrag after. Cleaning PC is possible manualy, but also with
a software called Ccleaner. Defrag is possible in windows itself. Right click
on your HD (C:) and pick "defrag" in the menu.
If the solutions above don't work, and it's a game related issue, I would simply reinstall the game
and check what it does. Good luck with it