This is a very familiar statement to me: I am 83 yrs old, started dizziness about 2 years ago; about the same time I was diagnosed with Polycythemia Vera; I have not undergone the tests that you mention but my symptoms sound exactly like yours. The quandary of dizziness is this: almost all medications have a potential side effect of causing dizziness; PV disease has a symptom of dizziness; inner ear problems can cause dizziness; eyesight can have an effect on dizziness; circulation problems can affect dizziness; the medical field obviously has no known way of detecting what is causing dizziness; I have learned to live with mine, but, I am a reluctant patient; I do not wish to spend my final time that I have left sitting in Dr.'s offices or constantly undergoing tests of one sort of the other.
My only suggestion would be a process of elimination, keeping notes of when, how severe or persistent the dizziness is, what you have eaten, what you were doing, how strenuous is/was your activity, particular stress situations, etc.
I have tried that, to some degree, but I must admit it has provided or proved nothing. It just is.
I wish you success and patience and if nothing seems to work; acceptance
Rosalie