Dysrhythmia left hemisphere

I am a fairly active 50 year old woman.  I have these spells similar to Vertigo.  When they occur, it is a though the world is rapidly moving away from me, or out from under me. Nothing like a dizzy spell.  They last from 5-10 seconds.  They have become more frequent in the last few months.  Some are 'stronger' than others, where I actually have to grab hold of something for fear of falling to my left or right...or dropping down as is the case when it feels as though the ground is falling out from under me.  No nausea, just a strange feeling afterward - almost disoriented, sometimes a headache, sometimes briefly tired. The results of 1st EEG, Dysrhythmia Grade I left hemisphere.  Results of Sleep deprived EEG, Dysrhythmia Grade I left hemisphere with phase shift left anterior.  MRI normal.  Echocardiogram and EKG normal.   Blood tests normal.  Doppler showed minor buildup in coratid arteries (previous smoker).  I take no meds other than estrogen and 81mg. aspirin.
Four questions:
1.  Would MRI detect ALL abnormalities in the brain?
2. They have recommended 'shift neuroimaging', is that different from a MRI, and if so what is it?
3.  What could be causing these spells?  
4.  I am having reconstructive surgery on my foot December 30th, should I reconsider until a definitive diagnosis is made?
Thank you.