Have you had your spark plugs replaced yet? Worn plugs can be the cause. Other ignition issues, such as a coil issue, coil boot issue (both for DOHC 3.0L V6 24V engine), ignition wires (3.0L V6 12V engine) can be the cause.
RepairPal Certified shops can diagnose and repair your 2002 Mercury Sable. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.