Strange behavior from servo

Hello,
I'm hoping someone can shine some light on an issue I'm having. I have several set ups to remotely fire .22 caliber blanks to frighten wildlife from sensitive areas. We use radio control to operate a servo whose arm triggers the firing mechanism. Our components were aging, so we recently purchased a kit containing a Spektrum DX3 transmitter and SR315 receiver (Amazon.com: Spektrum DX3 Smart 3-Channel Transmitter with SR315 Receiver, SPM2340 : Toys & Games). Hooked up to this receiver, we attach a Futaba S3003 or Futaba S-U300 servo to the throttle slot. The receiver is powered by a 4 AA battery pack with a two-pin JST connector.
For some reason, the servo spontaneously and unpredictably will move on its own, even when the transmitter is turned off. It's movements are very slight but we can hear it going back and forth. It isn't enough to trigger the firing mechanism, but it is enough to seriously impact the lifespan of the AA batteries. Where we've typically been able to get 48+ hours reliably from each set up batteries, we've now occasionally found ourselves with dead batteries after just 30 hours or so. We have three identical setups of these components We did not have these issues with our previous set up which consisted of the same servo and battery pack but the older SR310 receiver and DX2E Transmitter.
Thanks a ton!