Originally Posted by MJLavelle

Check the red connector that screws onto the bulb, and make sure the tabs that secure it have not broken. If they have, then they can be causing a bad connection.
Swap the bulbs around, and see if the problem switches to the other side. If so, it is the bulb. If not, it could be the ballast. Try swapping it to the other side, and see if the problem moves to that side.