The Stress Test and the Online play are completely different. I too am able to easily get 150FPS in CSS on the Stress Test albiet a lower end card (HD2900Pro 1GB).
Unfortunately I do not play it much, more of a TF2 fan. But I would suggest heading over to http://www.steampowered.com
and checking the CSS forums as I am sure there will be more people there who can help you.
Also check your rates. Sometimes they get messed up and cause lower FPS. Rememebr this as well, Source is a very CPU intensive engine and like the newer faster CPUs especially in the online games.