Coz if your using a router, you might have to update firmware. This is how i fixed it. Further more, try running gamebooster. My gaming rig is lagging so i would think that normal pc's would also be suffering.
Hmm. Well that used to happen to me.
To fix that, i just unplugged the power for my Linksys WirelessRouter, and Cable Router Box thingy. i plugged them back in 5 minutes later, and then disabled + Enabled my internet connection on my computer. (Wired connection)
If you have any problems then let me know.