Some timers won't work well with magnetic ballast flurorescents. I found that even the digital Intermatic timers have a hard time with them, and they are supposed to be compatible. It looks to me like your timer is causing the problem. Also, that can happen if the light is too cold when you start it. Magnetic ballast fluorescents also have a minimum starting temperature - around 55 degrees F. as I recall, but they start badly at even higher temperatures at times.