The whole machine just friezes, I can see the Arduino blinking underneath but all motos stopped. Also the print progress indicators in RepG get stuck. The USB cable is fine, it is the same one I used since the beginning with two magnetic ends and the Mac never goes to sleep. So I suspect the Arduino stopping due to peaks in the power supply like you are suggesting. It happened since we moved to a new place therefore I am quite certain. I will try the power surge power strip.
Printrun looks great but seems to be lacking one feature I often use in RepG, the travel rate multiplier.