All Illustrator (and Photoshop) gradients are subject to banding. This is from the help menu:
• Use blends that change at least 50% in one or more of the process colors.
• Use shorter blends (no more than 7.5” in length for less chance of banding.
• Use lighter colors for blends and keep blends of darker colors as short as possible.
• Retain 256 levels of gray by using appropriate line screen for output (too high of a line screen for the device’s output resolution can result in banding).
Basically you are not changing the color enough or the gradient is too long. GOOD LUCK!