Arduino Disappearing from COM Port

Hi all,

I'm using an Arduino Uno with an Adafruit Motor Shield to drive two stepper motors through LabVIEW. When I first started this project, programming only in the Arduino IDE, up until now, with a completed project, I have always had occasional problems with the Arduino disconnecting itself. When this happens, it ceases to appear under COM Ports in the Device Manager, sketches cannot be uploaded, and obviously the motors do not step. I can always tell when the board is in this state as the green LED will blink rather than stay lit.

I noticed that it is much more likely to happen (inevitable actually) if I step the motors using the Double or Interleave type steps. It is rarer with Single step. I'm curious if perhaps these modes require more power, and it simply overloads the Arduino.

It also tends to happen when I just let the board sit plugged in for a while.

Also, when my LabVIEW program is running, but the motors are not stepping, the motors themselves are seized up, essentially trying to step both ways. I suspect this may be the cause of the issue, but I'm unaware of any Arduino code that will prevent this from happening.

The only solution I have found to this is to unplug both the power and USB cables from the Arduino, then plug them back in while holding the reset button, but this does not always work. This will not be a viable solution when I'm actually using these anyway; I need to be absolutely certain that my motors are stepping when I send them a command as they will be in an inaccessible room during operation.

I'm running Windows XP. My motors are 5.3V at 1.6A, and 11.3V at .44A.

If anyone could help me out, it would be greatly appreciated.
Thanks!