The three things to check first
- Screws too tight. Every joint should swing with one finger and almost no pressure. If it does not, back the screws off a little, especially the ones around the main pivot points. A stiff joint makes the servo draw too much current, and the whole arm gets twitchy.
- Servo calibration. These are 180-degree servos, not continuous-rotation ones, so each one has a limited range and a home position. Check that the base can point straight ahead at its middle, that the shoulder servo can reach fully back and fully up, that the elbow can fully extend, and that the gripper opens and closes freely. If an arm horn was attached at the wrong angle the servo will try to reach a position it physically cannot, and it will jitter. Unscrew the horn, move it, and reattach.
- Not enough power. Plug into a USB 3.0 port, not USB 2.0; it can supply more current. If the servos still struggle, add a 9 V battery on the barrel jack as well as the USB cable. The board takes power from both safely.
Still stuck after those three? Email support and include a short video of what the arm does.