Just as important as having low impedance batteries is to be sure and use heavy duty (large diameter wire) leads and switches. Do not use servo extensions, solder in longer leads if necessary, as the connectors themselves often provide a high impedance, especially after bing installed for a time and not disconnecte/reconnected occasionally as they do corrode, some more than others. Yes, digital servos and long leads can cause severe enough voltage dropouts (they only need to be a few microseconds of duration) can cause 2.4GHz systems to reboot. A 5 cell battery is no guarantee against this if the wiring and battery impedance is to large. Note the word IMPEDANCE which is the opposition to any sudden change in current, not just resistance and this can have a significant effect when high currents are pulsed like happens with digital servos.