FPS lag anyone explain?

It sounds like you are having an application that start running on the background and it's taking all the CPU so it doesn't matter the settings of the game it's still lags because the CPU is busy. You could try a CPU log software to see what is taking your CPU power so you can fix the issue.

Yes it does happen on both single player and multiplayer. although on single player my fps never goes below 80 with 9 other bots in the game (trying to replicate 5 on 5 matches). ive tried checking my applications log and nothing appears to be using to much of my cpu power. im really stumped with this one because i would of thought that a 2.00ghz dual core could cope with counter strike source when my old 2.00ghz amd sempron (on my old tower pc with 2gb of ddr1) could run counter strike source with no fps lag (sorry of that sounded irrate didnt mean for it to be just trying to explain the issue accurately as i can)

right guys quick update. i found a cpu log program and i found that audiodg.exe was using 35% of my cpu usage so i turned audio enhancements off and it quickly went back down to 5% with counter strike source using 50-65% of my cpu while in game. i am however still receiving fps lag with there being a constant for the lack of a better word jiggering although fps is up in the 120 mark for example players will be running and they will hop a cm back every half a second (this is on single player so internet doesnt interfere). i am offically stumped guys

ive tried installing the new drivers with no luck. core 1 temperature is read 42 while in game for 30 minutes and core 2 is reading 41. my memory usage is only 1.5 gig out of 3 gig. anymore hints or tips any one can give me because the problem still persists