What Causes Recurring Dizzy Spells In A Toddler?

My 4 year old occasionally gets dizzy spells that can last from a minute to half an hour. He's had them occasionally (once or twice a month) for over a year. He's had bloodwork done, MRI, ECG, brain wave scans and all negative. As I write this he's had another one in bed and refuses to lie down again.

They seem to have no relation to time of day or physical activity. Sometimes its when he's playing and sometimes when he's in bed. He describes it as dizzy in his head but no pain anywhere.