ORIGINAL: Pyolet
Can't find Futaba 9501 servo anywhere...an older digital maybe? Most Futaba digitals aren't rated for 6v. 4.8 volts only. Switch to JR 8511/8611s or try with 4.8v pack and see if that fixes your twitching.
Woody.
Actually....2/3 of the Futaba digitals listed on the Futaba site ARE rated for 6v.