I m 63 years old, female and have been experiencing lightheadedness off and on for about 1 year. At first it only happened once in a few months but lately it has been happening much more frequently. It comes on whenever it wants and I could be resting, working , sitting or standing. I have had an MRI, MRA, have seen and ENT and have had an annual eye exam and all have come back normal. Can you tell me what this could be?