Have you seen an ENT Dr. to see if you have some inner ear infection or sinus problem. I know when my sinuses are acting up I can get quite dizzy even while just sitting doing nothing. I'd definately check for those.
You said your BP was fine, but could it be at times it goes low and your not aware of it? Does it happen more when your hungry..low sugar? Talk with your Dr. as even if your BP or sugar goes low temporarily, it will happen.
I'm sort of leaning towards what darryl said.
BTW Snow...yes that is a word but it's spelled DIZZINESS..;-)