Z axis changing direction DURING print????

I have had my Mendel working successfully for a few months now and just today I had two prints ruined because the z axis started moving down during the print instead of up! I have never had this problem before and I can't find any discussions online about it. After I paused the print with printerface, I tried moving the z axis again and the z axis moved up just like normal again. Any ideas??

I'm running RAMPS 1.4, sprinter, and slic3r 0.9.7. Also, my pololu drivers are being cooled so I don't think it is over heating. I checked out my gcode in a gcode viewer and it appears to be fine so maybe its a mechanical issue? Has anyone ever heard of that happening or know what would be causing it?

Electrical noise from some nearby source?

Really? Could that do it? My printer is in the same spot it has been for the past few months and it's never done that before, so I don't know where new electrical noise would be coming from...

Poor connection on the direction pin for the Z axis pololu?

Can you swap the drivers over or are they soldered in?

My pololus are swap-able. I will try that out and see if it makes a difference.

When I tried swapping my pololus, I found some wires to my z motor that had a bad connection and must have been shaken loose from the vibration of the printer. All is well now. Thanks for the suggestions.