[Delphinus]

I guess the obvious question is, is this a new bulb? Or was it working fine for the longest time and then something happened?

What you describe sounds a little like what happens if you try firing a bulb intended for pulse-start ballast on a probe start. Sometimes it works, sometimes not althouh it seems to improve over time (bulbs must burn in). If that's the case, the obvious solution is to replace the ballast with the correct one, but you may get lucky if you try switching the leads to the lamp around, double checking all connections, tightening that bulb in the socket just a little more, and so on.

But if it was working fine, and then all of a sudden stopped working, then I suspect something else is at fault. Again double check all connections and so on, but it could be you need to replace the lamp or the ballast (or a component).