1. Have you defragmented your hard drive lately? From Windows (Start->Programs->Accessories->System Tools->Disk Defragmenter).

2. Do you have this problem when not running wow?

3. Did you check your Windows Event Viewer to see if there are system errors? From your desktop, right-click your My Computer icon. Highlight and click manage. Drill down to Event Viewer. Look in the Application and System event logs. Might find something there.