It could be that the file has some bad points in it. If one of those points is a bit out of line, say it's in the middle of the block while you're riding on the street, the 500 would think you were supposed to turn and you didn't. Or it could be the 500 getting poor reception.

I'd start by making sure the 500 is set to record every second, then zooming in on the track it records and looking to see whether it thinks you were where you really were. That should tell you whether it's the device or the file.