Is that a 9V smoke alarm battery (PP3) powering the servo? If so, that very likely may be the problem. Not only is 9V too much voltage for a servo, those batteries can not supply very much current. When you try to pull too much current the voltage will drop, like you are seeing. A 4 AA cell battery pack is a better supply for a servo.
PP3 batteries should have a warning on them, "Do not use for Arduino projects".
This tutorial is a starting point on how to draw a good schematic.