I've been doing a bit of research on this, since delayed bloodflow to the brain when standing and dizziness is a common MS problem, but as sou said, it may not be due to lesion location, but more likely, vascular issues. Go to www.ccsvi.org
if you're not familiar with the recent gains in research regarding MS and the vascular system.
One of the areas I started looking into was hypovolemia, or low blood volume. I began studying this because of the high restenosis rates in CCSVI angioplasty, jugular collapse syndrome, and the fact that Dr. Dake kept insisting that my husband Jeff make sure he was hydrating. Hypovolemia occurs in people who are not hydrated enough or inactive. It happens in the elderly, and the chronically ill. It also happens in those who lose weight quickly. (been thinking about this for you too, Gibbs.) And hypovolemia is a cause of orthostatic hypotension. Here's a whole thread on hypovolemia...
http://www.thisisms.com/ftopic-15073-0- ... rasc-.html
One thing to look at is how much H2O you drink each day. Jeff's upped his liquid intake and it's helped keep blood flowing and all system's go. He also stays very active. Try it, it might help....hope you feel better.
Here's a handy calculator I found to figure daily liquid requirements:
http://nutrition.about.com/library/blwa ... ulator.htm