Arduino can't provide that kind of power? I won't check that until tommorow but it's hard to belive in for me. Especially that the specification of the servo (HS 645MG) says that it operational voltage is between 4.8V and 6V (changing the torque respectively) and the servo moves as it's supposed to: the problem is that it blocks the serial communication.
Any other suggestions?
What is happening is that the Arduino is repeatedly power cycling because it cannot provide enough power for itself and the servo. When the Arduino loses power, it loses all communications with the serial port. The MORE SERIOUS problem is that YOU WILL DESTROY the Arduino if you keep treating it like this.
You don't HAVE to believe what those of us with DECADES OF EXPERIENCE in electronics tell you about this. You don't have to believe what those of us that have used Arduino for YEARS tell you about this. Your choice.
Since you are so certain that NONE OF US know what is happening, let me just leave a couple of things here for you:
From the Arduino website:
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
From the ServoCity website regarding the HS-645MG:
Current Drain (4.8V): 8.8mA/idle and 350mA no load operating
Current Drain (6.0V): 9.1mA/idle and 450mA no load operating
Now I'm just a simple man, but in MY WORLD a device (the Arduino) that can provide 40ma is NEVER GOING TO FUNCTION PROPERLY when trying to power a device (the HS-645MG servo) that requires AT LEAST 350ma when operating. That means that the servo requires 8.75 times MORE current than the Arduino can provide.
But hey, it's your right not to believe this. PLEASE let me know once you have managed to change the laws of physics so that your hypothesis will work.