The 3D Printer controller boards typically don't have a lot of extra power. Using the +5v of the controller board to power the servo has caused problems for a lot people. Usually, the answer is to put a 5 volt regulator on the server. Specifically... get a 7805. It is 3 pins. Connect the ground pin to ground. Connect the +5 volt output to the servo's power supply wire. And connect the input of the 7805 to a +12 volt line of your power supply. At that point, you won't be taking power from the controller board. The servo will have its own power and probably the problem goes away.

You are not the first person to have this problem!