Had this problem when all high power digital servos used
Even with a fully charged 6v 2500mAh (AA) batteries
Found this when I was bench testing
Even in same room, after awhile with radio on it would fail safe

Digital servos seem to draw a lot of current even at neutral
With the 2500mAh 'AA' batteries and 5xHitec HS5645 servos, it barely lasts a minute before they cannot maintain the current demand
sub 'C' cells last a bit longer, but I always use Li-Po and regulator with any digital servo setup now

I've had the Li-Po(2200mAh)/Reg setup and left radio on for an hour with no problem