I would calibrate you board. You can do this in Measurement and Automation Explorer (MAX), but you have to use DAQmx and not Traditional DAQ. To do this, launch MAX and on the lefthand side expand My System>>Devices and Interfaces>>NI-DAQmx Devices, right click on your 6025 card and select self calibrate. This should only take a couple of minutes. I think this could fix the offset you are seeing.
You can find the default settings by right clicking on your device and selecting Properties. I don't think that your card is initialized to its default settings when you reboot LabVIEW.
Hope this helps.
JenK