What Is The Possible Cause Of Periodic Lightheadiness Sort Of

What is the possible cause of periodic lightheadiness sort of dizzy but not spinning. have had these spells off and on for several years. Have had heart tests, an MRI of my head and other tests with no cause found.
What are we missing. I am in good health otherwise, passed a physical and had gall bladder surgery in Oct.
I do have arthritis and have had a knee replacement 4 yrs. ago. the only medicine I take is Nexium.
What do you suggest???