there are dozens of threads on this topic. My experience is that Samsung's GPS restore followed by a reboot usually fixes it (though never as sensitive as my Garmin GPS, especially in dense cities).
Occasionally I need to delete the gps aiding data that has accumulated, (use a GPS test app for this).
the problem is mostly software - sometimes the gps gets signal from 10 satellites without locking on, then they gradually disappear, leaving only one or two.
Was recently in Europe, and lost the SD card from my Garmin with European maps - the Vibrant was surprisingly good at getting me around!