johnwasser:
The USB cable provides a fairly good ground that a battery won't. Perhaps some of your inputs are floating and you are getting more noise without the Ground.
I am using an external DC power supply (that is converting from 230AC).. I guess this has a good ground?
I just tried different code (simple servo sweep, both servos at the same time) and the same problem arised (so I am shure it's not the code)...