T.H.
Okay, I thought this was nuts when I heard it too...but there is apparently some disorder that's called sporadic fever disorder or something (that's probably not it, but if you google something of that nature, it should come up). A friend of mine's daughter has it and basically she gets a fever about every 3 weeks. My friend started charting it on a calendar and just tried to do some online research and she found it...it seems that is what the doctor thinks it could be too. Also her husband is a pediatrician and he couldn't find anything else wrong with her either.
I don't know much about it, but it seems fairly harmless in terms of disorders. I would see if you can find out about it and ask your doctor. If your ped. can't find anything else, maybe that's it!?!