Problems with Counter Strike Source

Hello Everyone, I am having problems when I play Counter-Strike Source. It seems to be skipping or lagging every 30 seconds or so. Sometimes less then 30 seconds and sometimes more. It seems like lagging but when I check my latency it is perfect with no spiking at all. I play on another computer in my house and it doesnt do that. It is more of a skip not a lag. At first I thought it might be my graphics were to high. So I turned my settings all the way down and it still did it. I have played this game for years and it never did this till now. I decided to upgrade my graphics card and it still didnt help. I had 1 gb of ram so I decided to buy 2 more. So now I have 3 gb and it still does that same thing. I have turned my refresh rate all the way up on my monitor and it doesnt work either. This game has always worked until now. I thought reformatting my computer would help but that didnt. I bought a new hard drive and it still skips in the game. I have downloaded all the new windows updates and driver updates. I went to my video cards website and downloaded the most recent update. Still NOTHING. Can anyone help me please or offer any ideas.
Thanks