Ultrasonic sensor returns random values on external power supply
So i tried to used a stable 5V input on the 5V input pin and got the same more or less random values (jumping all around... especially on targets more than 1m away) 

So I changed form newPing lib (http://forum.arduino.cc/index.php?topic=37712.0) to Ultrasonic lib (http://forum.arduino.cc/index.php?topic=37712.0).

Now the values are correct on each power supply and even more accurate on high distance than with newPing on USB power supply.

Don't know whats broken with the newPing v1.7 lib -.- but this works!