Games stuttering

So I'm not sure if this is the right forum to put this in since I'm not positive that it's a gpu problem, but oh well. I just recently built a new system, and here are the specs

E8500
Gigabyte UD3R
GTX 260 core 216
G. Skill Pi Black 4gb ram
Corsair 650w psu
XP

I think that's all that matters for this post, but let me know if you need more. Here is my problem. I have mainly been playing Left 4 Dead, Farcry 2, and Warhammer Online on my computer. I didn't have any problems playing FC2, which was the first game I played, and it looked great, but whenever I play L4D or Warhammer, the game will freeze for a few seconds, or at least stutter and lag at certain points in time. It is much worse in L4D, and it is in both single player and multi-player, so I don't think it's a connection problem. I was running it on maxed out settings, but lowered the settings and still had the same problem. It should probably be mentioned also that I got both of those games off of Steam, could that have an affect on my games performance for some reason? I am really disappointed because I built this system so I could play some great looking games and have them look and play great as well. I know there aren't many specifics, but if anyone could give me some tips or help, I would be very grateful. Thanks a lot everyone.

how hot is the 260? try underclocking it by 50mhz on the core and mem. probably won't fix it, but its worth a try

That seems to have fixed it. Thanks a lot for the advice.