What Causes Severe Dizziness, Heaviness In Eyes And Tiredness In An Elderly Person?

My 83 year old mother is suffering from: bad dizziness; heavy feeling in her eyes that she cannot move them from side to side quickly as she feels she might faint if she does; overwhelming tiredness. This has been going on for 4 years and has gone undiagnosed after seeing neurologists, ophthalmologists, cardiologists and otologists. All diagnostics, CT SCAN HEAD, MRI HEAD, X-RAY HEAD, AND CAROTID ULTRASOUND are negative. She is starting to lose weight and is getting worse. Prior to developing these symptoms, she used to walk 5 km. per day, and was a work horse around her house and garden. Now she can t even go downstairs due to a fear of falling down the stairs. Do you have any suggestions?