I periodically will have a pack that does that, and once it starts the imbalance grows. I made a special charge harness that lets me charge individual cells through the balance plug so I can manually bring the whole pack even again, then I do a deep cycle with it, and everything is back in harmony.
I've been doing this to maintain my pile of Turnigy 3S packs for a couple years now, and I have hundreds of cycles on these packs and very few failures. I am still able to get the rated amount of power out of them without going below 3.6V per cell, so I guess its working.