Maybe you have a loose plug? Or one of the cords has been stepped on & now has (internally) a bad connection? Or the bulb has a defect?

Is the light regulated by a rheostat (lamp dimmer)? If so, & IF the bulb IS being dimmed, you might have power dips that result in not quite enough power
to keep it on. (power dips are out of your control...that's the power company's department)