Some servos are extremely power hungry like Hitec or JR. Example:
Protec UBEC with warning LED that goes on when pack voltage goes bellow 6.2V
Fully charged 1500mah LiPo rx pack who knows what rating
Nitro buggy on a car stand
Servos at full brake and full steering (EPA set right
)
With either 2 Hitec (one 56xx one 59xx) or 2 JR servos (9100T and S) the warning LED goes on so servos must draw quite a lot of Amps to drop from a fully charged to under 3.1V per cell.
Does not happen with Futaba Brushless servos
And since RX8 has "just" 5A bec it may be overloaded by power hungry Hitec servos causing issues.
So either run RX pack or get less power hungry servos like Futaba BLS line.