Serious LAG problem in video-related apps.

Ok, well my problem is that everytime I try to play any game online (or single player...) I'll play for about 5-10 mins and i will get this severe lag problem where I lag for a straight 10-15 seconds straight, nothing moves, whatever sound that was playing before i lag keeps playing over and over again, my whole system just plainly locks up. I only seem to get this problem on my desktop, my laptop runs on a 2.0 ghz proc, 32 mb video card, and 512 ram and seems to work just fine. (on games like Counter-strike 1.6, which is the game that i play normally) I normally ping pretty average 20-40 (like i said...in counter-strike 1.6)
and my fps doesnt usually go under 80. But like i was saying, it goes at first for about 5-10 mins, and after i get the first lagging session, it lags even more often the more i play. I first thought that it was my video card going on me (I played on a ATI Radeon 9200SE 128M DDR) So i put my original card in from when i got the computer (which is a NVIDIA GeForce MX4000 128M)

I did that a couple days ago and it fixed nothing. So im not sure if its something with my motherboard or what...(Im sorry, I cannot rightfully list the brand n all that of my motherboard, but i can say that I run on a 1.70 ghz processor, with 512 ram, and i listed my video card earlier...uhhh my computer itself is a Compaq Evo Desktop..and the motherboard is the original one that came with it, if that helps any much at all...)