When to replace
Inside a fluorescent light, electricity is delivered to a ballast, which sends a spark through the mercury-vapor-filled tube (or bulb), creating light by activating phosphors that coat the inside of the tube. If any of the components are faulty, the light won't work right. Once you pinpoint the problem, most fixes are very easy. Remember to turn off the power first!
Old bulbs blink off and on just before they die; repairs are often simply a matter of replacing the tube or bulb. This problem may also be occurring because of poor contact between the pins at the ends of a tube and the tube holders. If the pins are bent, use needle-nose pliers to straighten them. Clean up the pins and socket contacts with fine sandpaper, and brush away residue.
Though gray bands at the ends of tube are normal, black bands indicate that the tube needs to be replaced. If only one end is dark, turn the bulb end-for-end. If the tube is new and these fixes don't work, you may have to replace the starter or ballast.
Some fixtures ("A-rated") are quieter than others are, but most fluorescent lights have a slight hum. If the sound seems too loud-- or if you can smell electrical burning-- shut off the power. The ballast is probably either the wrong type, improperly installed, or defective. Replace it or call an electrician.
Brand new tubes tend to flicker, as do bulbs that are cold. If an old tube still flickers after it has had a chance to warm up (or after you've warmed up the room), rotate it a couple of times in the tube holders. Try cleaning the tube's end pins. If it still doesn't work, replace it.
If only the ends of the tube glow, either the starter or the ballast is defective. Replace the starter, then the ballast.