I have similar problems, gps position being completely off. Except at hone when I installed it first time, it found the coordinates, 20m off by the way.
Does the auto calibration gyro thing help to get matters sorted?
I resetted this cal. setting and looks like it needs to learn now. First spin around the block showed me driving through the neighbours houses then ended up in a field....
Could it be I need the metallic sheet under the gps aerial, it is mounted just two cm above the f920BT unit, hoped this would be enough shielding.
In my setup I haven't connected the speed pulse by the w