Have you ever done driver updates for your machine? Primarily the graphics card drivers. Those can often make or break your gaming experience, if you have old drivers, such as what shipped from the manufacturer's factory.
You can find updated NVIDIA drivers for your machine here:
http://www.nvidia.com/Download/index.aspx?lang=en-us
Next, are you using a wireless connection to your internet? If possible, can you please try using a cord instead? I have seen random lag spikes at times when the wireless signal is flaky. Removing that from the equation will help in troubleshooting.
Also, it might be worth looking for driver updates to your networking device, both the wireless and the wired, and see if that helps.
Are you going through a router?
Can you possibly connect directly to you modem for a short period of time as a test, to see if your router is causing issues?