Maybe the extra force from the higher head speed and extra torque is overloading the servo's, or maybe the motor is drawing so much electricity it's giving bad power to the servo's or something. You could check how hot the pack is... if it's really warm/hot then that might be the problem, and it would be more likely if the pack is old and you are getting short flight times on it. I dunno how it would factor in big picture overall though....

I guess I forgot to mention how unlikely this is [8D]