This might not apply to you. But as you know I was lagging really badly when I first moved down to my parents house from Athens. At first I though it was the cable connection. Well I upgraded to 3Mb and it was still just as laggy. Next I thoguht it was my dad's shitty router...so I plugged mine in and had the exact same problem still. Finally I turned off all my firewalls (windows xp one AND Zone Alarm Pro)
The answer was Zone Alarm Pro...I'm not sure why it was causing my ping to spike so often (it worked fine in Athens) but it gave me the exact problem you are having. So now I just turn Zone Alarm off before I begin playing and everything is fine.
before that (when I was still in athens) i bought the above mentioned router and had a problem similar to yours. I called tech support and told the guy I was getting lag spikes trying to play CZ. He told me how to optimize it for online game play. So if it's not the firewall...might be an improperly configed router.