gandalf50:
I solved it!External power is needed to the Arduino, i.e. just powering via USB is not enough to get good results when multiple ping sensors and an Ethernet shield are used.
This is regardless if an Mega or an Uno are used, or if the PC have 1 or 2 PSUs.//Magnus
Interesting! My ultrasonic sensors only use a couple mA each, while the Arduino Uno uses 60 mA (I'd guess the Mega uses more). I wouldn't think that just the sensors would put your USB or the voltage regulator over the top. Have you measured what kind of current it's using? It just seems very unlikely that it's within a couple mA of going over the top and the sensors draw enough current to do it. I guess it's possible, just doesn't seem likely.
Tim