Yes, the bulb may be suspect.
Inside the bulb (assuming it is the stock type) is a wire filament. Well, actually two. These filaments sometimes break at one end. But, then dance around and intermittently weld themselves weakly back into position. Then vibration breaks them again and the cycle can repeat itself for a while.
You could also have a bullet connector socket in the circuit path that is loose and being intermittant, too. If you haven't checked for that, already.
