I would say from your description that one of the cells is dead. This happens because the cells are not matched when the pack is assembled. The pack fully discharges and causes the weakest one to fail.

Check out the receiver packs on the Stickman RC Products website. They are matched and hand soldered. You can get them at www.amainhobbies.com and www.modhobby.com search stickman from their search box.

www.stickmanrc.com