First, I would try discharging it until it turns itself off then fully recharge. See if this makes any difference, I'm guessing it won't but maybe worth a try.
Secondly, if it is still under warranty, I would send it back, I'm guessing it's not though, so...
How confident are you about taking a battery apart?
Those cells need balancing. There are two ways of doing this, charging the cells with lower voltage or discharging the cells with higher voltage.
If you want to try this, let me know and I'll talk you through it, I will take one of mine apart and post step by step pics.