Sounds like somthing is running in the background (virus / adware) , sucking up resources ram/paging file/cpu, I have seen quad core cpu w/8 gig of ram run like an old 486 with 4 megs of ram while under the control of a nasty bug,Only fix is a reinstall of OS
If you in the past had no issues and now you do this is prob your issue.
Unless somone at home is on another puter downloading movies/tunes at the same time , while playing the game give it the whole bandwidth you have.