I too have been suffering from this bizarre running issue.  I have been running for 11 years I have run 10 full marathons countless half's and 10ks.  Throughout all the training for these runs and on race day I never had any dizziness or nausea.

One day, about 14 months ago, I was out for a 12k steady run and I became dizzy around the 3 k point.  The dizziness increased to the point I felt very nausea.  This happens almost every time I run around the 3k point. 

Sometimes it is worse then other times.  It will go away after sitting for awhile but there have been 2 or 3 times where ity has lasted 3 or 4 days.  These times I pushed through and ran my training distances of anywhere from 23k to 30k.  Being ill for 3 or 4 days after a long run has taken ALL the enjoyment out of running for me.  So I have stopped running altogether.I don't even go and run short runs now as it just fustrates me and really whats the sense I am going to feel dizzy and ill within 20 mins of beginning.

I do bike and swim and these sports do not trigger the dizziness or make me feel nausea.Nothing else makes this happen.  I do not get car sick or experience this when doing anything else but running.

I have had a MRI on my brain, EKG, Cardio tests, seen a ear/nose throat specialist, had blood work completed and they say there is nothing wrong.  I am in very good health.

No one knows why this is happening when I run.  I wear a heart rate monitor and heart rate is exactly where it should be even when the dizziness is in full swing.

The doctor told me to take Gravol when I run which cures the nausea but does nothing for the dizziness. I have been tested for BPPV (I think that was what it was called) and I have been told this is NOT what I have as I don't have it all the time.

It seems there is enough of us experiencing this...how can we attract the interest of someone who maybe able to help us?