For those that have been following this thread, I have some conclusions based on comments from other readers and further research on my part.
Conclusion #1: Clouds do affect GPS reception. At least Phantom reception. Not all clouds however. Those puffy white things don't seem to have any effect on the reception. However, dark clouds that cover the entire sky dome do affect it. I went to the same field for a few days. Overcast one day, 6-8 sats. Clear skies, 10-12 sats. I repeated this test twice with the same conclusion. Within the course of an afternoon, the heavily overcast skies cleared up, and my reception did also. I think it may come down to each of our specific locations however, as to how much this affects each of us.
Conclusion #2: Flying near a cell tower does not affect the Phantom. Not sure if you get right on top of it, but I saw no difference when flying in a field miles from a tower and one sitting 100 yards from a tower.
Conclusion #3: The foil mod, as it is referred to does help. Basically, the Phantom's GPS is a real disappointment to me. Something as complicated as the Phantom, and as reliant on good GPS reception should have a better GPS configuration. One that doesn't require us to tear the unit apart to try to insulate it better. Having the GPS antenna located where it is helps ensure the aerodynamics of the craft, but its just too close to all those leaky radio waves.
Conclusion #4: The foil mod does require the use of aluminum and not copper. I had done mine with copper sheets that are used to insulate humming in a guitar amp. I saw no change in my reception. I pulled it out and used aluminum. Big difference. Although copper is a better conductor of electricity, apparently, aluminum is a better insulator of EMI (electro magnetic interference).
Thank everyone for their comments. I hope I did not offend anyone with my conclusions. My next step is to ditch this GPS antenna system.