Counter-Strike: Source was recently moved over to the Orange Box engine, what you are describing is exactly what people who have been playing Team Fortress 2 have been dealing with since launch. The Orange Box Engine is horribly optimized and has driven FPS numbers down with every single update. As an example my current PC can max Crysis but I cannot pull 60+ FPS at all times while playing Team Fortress 2, Counter-Strike, etc.
Your best bet is to go through the Video settings and disable the Shadows, HDR and if the Multi-Core support option is included with Counter-Strike now make sure it's enabled.
Hopefully this has helped you.