I think you might've gotten bit by the "more mAh == better" fallacy. Different batteries have different internal resistances, but there's no way to tell except by testing. The higher the internal resistance, the less Amps the battery can produce. You have to look at how many Amps your motor setup can draw, and choose a battery that can produce that many Amps.

According to Hobby Lobby's website, the recommended 1700mAh pack for the Pigi 3D is good for 35 Amps peak short-term max, while the 2150mAh pack you chose is only good for 20 Amps peak short-term max. From skimming the Pigi thread linked above, it seems that with the recommended PJS1000 motor, it draws in the mid-20's for Amps at peak. That's more Amps than the 2150 cells can produce, let alone produce on a constant basis, and they knuckle under. Voltage drops, Amps drop, battery gets really hot, plane flies like doggie doo, customer is unhappy...

Don't give up on the plane just yet! Get the recommended 1700mAh pack, and you'll have a whole different plane. Talk to the guys at Hobby Lobby; perhaps they'll take the old battery back if you explain to them that you bought it for the Pigi by mistake.