Try putting something like a harbor freight dimmer on the light and slowly turn it down until you get the flickering you want. This is the best way but I can't guarantee you're not damaging lamp and/or bulbs to get this effect. You could also try a "magic gadget"
which is basically a device to help you control a flickering effect. They're normally used on regular hot lights to get firelight effects. You can use your imagination on how they would work in your instance. Again, I can't promise that there won't be some damage to your ballast and/or bulb. I read about someone using this device somewhere else but he said its touchy and you have to find the sweet spot that's just enough to get the ballast to fire.