Some herpes viruses are activated by the sun. Could it be your weakened immune system is having a hard time holding down these viruses that people without CFS usually do not have a problem quarantining, such as EBV or Cytolomegalo?
I think this is exactly right but I think the virus in question here may be HSV1.
@helsbells - Do you suffer from oral ulcers by chance? Are you HSV1 positive ? Your symptoms sound strikingly similar to Behcet's Disease to me (which I believe can be triggered by HSV1).