if you are sure you do not have a vacuum leak, you may have a failing air/fuel ratio sensor. it is failing common on the toyotas. use a toyota sensor, the aftermarket sensors may not give the computer the correct values needed to keep the light from resetting.
RepairPal Certified shops can diagnose and repair your 1999 Toyota Camry. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.