Perhaps you should check the internal battery on the external gps antenna. The battery allows the antenna to maintain the last fix until the unit is powered up again. I have also read on this site, in recent posts , that a dead battery in the raystar 125 antenna prevented the gps from acquiring satellite fixes as well as low battey power causing erratic readings.
Note that there was nothing in the owner's manual about the internal battery being present in the gps antenna. The bottom plate of the antenna needs to be removed to gain access to the battery. Might want to search the archives for further information. Hope this helps.