The behavior that you are describing sounds like what I might expect with too little current, or too high of a rotation speed. If you have adjusted the rotation speed in the extension, try setting it to 300 as a baseline value.
Questions for you:
1. Are you using the standard motors and power supply that came with the EggBot?
2. Does your EBB have a check mark on one side of the terminal block?
3. If you disconnect the motor wires from the EBB, can you turn the motors by hand? If so, does the motion feel smooth and continuous (with very fine grain light cogging), or does it feel rough, stuck, or variable? If the latter, then your motor(s) may have been damaged in shipping.
