Odd indeed.
Next time you are running it, keep an eye on the USB and status lights. Does the behaviour change while its running in some noticeable way? It sounds almost like the power supply may be fluctuating enough to reset or confuse something.
Also, check the output voltage of the supply if you can. When the motors are running you may see the voltage drop if the supply is struggling to give enough power.
For the north axis, check that the pulleys are not slipping - have the grub screws worked loose? Also have a look at the backlash in the mount - can you spin the pulley back and forth a long way without the mount moving?