Are you getting any kind of error message on your computer?
We have heard of this happening in several different case like this (but with an error message) when someone was using a USB extension cable.
If you are not getting an error message, then it might be the case that you have an intermittent power connetion. If the power is disconnected (but not USB), the motors will stop working but you will not get an error message. You might try wiggling the power cord while printing to see if that makes any difference.
