99% it is a bulb flickering because its at the end of its life but another can reason can cause it to flicker on+off if your using electromagnetic ballasts (not the digital/electronic type), in these there is a thermal cutout which when the ballast doesnt do its job in limiting the flow of current due to an internal fault with itself and lets the current run away which would destroy the bulb, the thermal cutout operates and opens the circuit when the ballast gets too hot (hot enough so it blisters the skin to touch it). Once it cools down depending on the type of ignitor you have it can light again, go out and so on. How you know that the thermal cutout is working without touching the choke it to look intoo the bulb when it restrikes, if the bulb gets bright that you cant look into it in after a matter of a few seconds then it the ballast is the problem. It can happen and catch people out very easily when using electromagnetic gear but is rare thankfully