hey Chi3f, I beleive it has everything to do with the batteries -under load, if the batteries cannot supply sufficient current, you get a voltage drop in the pack and thus insufficient voltage to the servo's if thei are powered by the BEC on the esc. Running a power cap will as you say help as it will discharge when the battery is unable to provide all the juice the system needs (its an extremely low resistance energy source). either way, your best bet is to try a power cap (and be careful there are 2 different locations you can install them, one on the battery input wires (this has the same effect as increasing the burst C rating of the pack) or onto the receiver itself which will help smooth current fluctuations during high demand situations... I personally use both...
Take care,
Demon