All I can say is good luck. I've had just about the exact same symptoms for years with absolutely no explanation, no pattern, etc.
I didn't test positive for anemia, but eating enough iron does help. Going to the chiropractor also helped, a lot. Are you more stressed than usual, and do the muscles in your neck feel tight?
With me, it happens kind of on and off. Sometimes I feel like that for the better part of a week, then it will only happen once for an hour or so in the next few weeks, I'll go three months feeling fine, and then I'll feel it every few days for another month or so.