Have you seen calculations from last post? Your battery level seems absolutely fine. Especially, that I've considered fully-fresh battery, and as You've said, You're using one in poor condition.

If you're not satisfied with lifetime of standard battery - which is understandable - consider doing like me, and use mugen cover + DIY/bought dual-scud, which totals up to little more than 3000mAh capacity. Or wait/donate/pre-order to body replacement, which I supposed to have room for 2x 3,4Ah 18650's, totalling with 6800 mAh capacity.

In any case, it doesn't sound to be U-boot problem, at all.

/Estel