What Is The Cause Of Lightheadedness On A Regular Basis?

I undergo "Ligtheadedness" off and on daily. This has been occuring for 2 months. (started on Mar. 13, 2011}. I've been to my Cardiologist, my ENT Doctor,a Neurologist and my Family Doctor. They have done thorough blood work, and MRI s of my head. They have all given me a clear bill of health. I am so frustrated!!!!!!!!!!!! I need CLOSURE.

PLEASE HELP ME ! ! ! ! ! My E-Mail address is benxing@att,net. I am a female age 65+ ,5'4" tall

and 135 lb. I had 2 stents put in 3 yrs ago.