I suspect your voltage is too high for the servos used. I manfactured model servos for 16 yrs (1969-1985) and if any one ran more voltage than specified all warrantees were voided. I recently contacted Hitec about running their servos on 7.2V (2 A123 in series) and got vauge answer. They stated you could run their standard servos on 7.2 but I didn't find out which ones are standard. Personnally I won't run any servo above the specified voltage.