Just a guess here. Was your father in the service or has he traveled to a malaria-risk areas? Malaria came to my mind because your fathers symptoms sounds a little like what my husband went through years after contacting malaria in the service. People who have had malaria can have relapses and suffer several additional attacks after months or even years without symptoms. Relapses can cause fever, chills, weakness and flu-like illness. Attacks consists of a cold stage (sensation of cold, shivering), a hot stage (fever, headaches, vomiting and a sweating stage (sweats, return to normal temperature, tiredness).
However, I may be way off.
Interesting because when he gets these spells he calls it his "maleria spells" since the symptons are similar.
Yes, my father did travel in the service over 40 years ago. He has been having these spells for over 30 years, once a year.
This year it was twice and he fainted 3 times in one day and sent himself to the hospital.
I believe there is treatment to reduce the chance of relapses.