The stock Spektrum pack is made up of four A cells. You probably bought a pack made up of four AA cells. A cells typically have a much higher current drain capacity than AA cells. I've used AA cell packs in my planes that have standard servos, but I get four or five A cell packs for my planes with high torque servos, and for any planes with a Spektrum receiver in them, so I don't have to worry about a brown-out reboot of the receiver.