A cyclic pattern of fever is seen in EBV or infectious mononeucleosis, malaria, autoimmune disorders like rheumatoid arthritis, SLE,
otitis media, endocarditis, IBS, STDs and cancers. Rare causes of unexplained fevers include (but are not limited to) taking certain medications, adrenal disease, hematomas, hyperthyroidism, pulmonary emboli, and previously undetected thrombophlebitis You can go over all these possibilities to see what has not been tested for.
Hope this helps.