Since you've done what other ppl have suggested. 1) Hard drive. Could be failing, could be fragmented, could be not enough free space left for virtual memory. 2) Might want to bust open the case and clean out the dust in your computer. A dusty GPU will lead to a higher temp and unstable game play. 3) Next time your computer lags, make a note of the time and check Event Viewers. Control Panel -> Admin Tools -> Event Viewer. If you find errors, Google the event ID for further info.
