I've had dizzy spells since college - they often seem to be related to a combination of sleep, food, and stress. If I'm not sleeping well, hungry, and have low-grade constant stress I tend to get them.
I'd been to neurologists and regular doctors without any diagnoses. During pregnancy #3 I had a problem with my vision, and the opthamologist recommended I be tested for a couple of blood clotting problems. Turns out I had one of those.
The one I had was called MTHFR - it's a genetic issue that means your blood's more prone to clotting than other folks. This is something nearly 15% of the population has, but it's not usually an issue. However, if tiny clots get into your brain, they can cause dizzy spells.
You might want to talk to a hemotologist if you're interested in being tested. There's also a thing called ATM deficiency (or ATP? It's been a while) and they diagnosed that first, then undiagnosed it when I was over being pregnant and my levels went back to normal.