Well, many of us have tried with different custom rom's or system specific tools, but with no luck. And i'll tell you why, because its an HARDWARE issue, check this out:
All you have to do is open the back cover of your phone and to the upper right (near the speaker) you'll find the GPS antenna. It is covered with some kind of black plastic tape.
Now, just apply some pressure to the very top of the antenna with your finger (the area marked with the red circle), not the middle or bottom, that makes it worse.
Open a GPS test app ( i use GPS Test Plus v1.2.1 - mirror) and check how fast you'll get a GPS signal / fix. You can try it before and after, to see if there's an improvement for you.
If it works on your phone, all you need to do is simply attach a small piece of double sided tape or similar to the top of the antenna so that when you put the back cover on again their is pressure on that spot.
Hope this will finally solve your Samsung Galaxy S GPS issue.