futaba 9C and 6 volts?
How many servo's are you running? Have you checked the voltage drop after testing the servo's one at a time and seeing if you have a bigger drop on one? It could be a short somewhere, or a couple servo's could be binding at full travel. What capacity battery are you using? Bigger capacity cells tend to have less of a voltage drop (to a point) You may want to try running two packs in parallel. That kind of a voltage drop is pretty extreme.