I was thinking that there may be a fault on a disc that installs a file incorrectly onto your hard drive.(A long shot)
run dxdiag if you havn't already and see if there are any hardware or DX conflicts.
You can also go to Drive C right click and go to properties tools and run a scan to correct or fix any errors on the drive.
If your video card is dirty it could cause overheating and poor performance as well.
You can also run msconfig and see if there are any applications running in the background that might cause any particular graphically intensive moment during a game to lag.
Have you recently updated to any new video or sound card drivers?
If so, try rolling them back to see if you notice any difference.
Those are my suggestions. I'm not a tech but after living online and gaming for years and years these are things that I have picked up that helped me.
I hope you can sort it out, good luck