I'm afraid I can't help much there, you'll have to wait for more people to show up on the new forum. I would suggest checking to see if anything shorts (perhaps a brought out pin header on your gps shield touching the bottom of the xbee shield?)
Also, this might be happening when you flip the xbee/dline switch - try reproducing this error with the arduino -->xbee --> gps setup, that way you can be sure it isn't electrical.
Or, maybe your gps is getting interference underneath the xbee.