Likewise: my backlit portable has always had a few brightness "steps" that are no-go areas because they make the backlight flicker slightly. It seems to vary a bit with temperature too - after the machine has been on for a while, the problem diminishes.

However, the machine recently developed a nasty habit of blinking the backlight on and off. Am in the process of replacing the electrolytic capacitors on the logic board - hopefully this will fix the problem. The backlight inverter board has a couple of electros on it, but both are in good order and the heavy lifting appears to be performed by a polyester cap, so i doublt the problem is on the inverter board.