This looks identical to the problem I'm posting about here:
http://forum.arduino.cc/index.php?topic=238351.0
While it might be a software issues (Strings, etc), my feeling is that there's a hardware problem. Are you using a USB hub?
Tomorrow I'm going to try with and without a USB hub to see if I can identify the source of the problem.
My other thought is that there's an issue with power/ground....