CFS is a complex disease :( There are many things to look at, for instance I would begin taking magnesium (high quality supplement like UltraMag from Source Naturals) See Mildred Seelig MD research on magnesium and chronic fatigue. Blood testing for magnesium is not a true indicator for total magnesium stores.
You might want to consider looking at the possibility of heavy metals - use Doctors Data heavy metal challenge for this test.