Thanks.  I'm wondering if there are extremely common causes though, before I take more drastic measures.  For example, I've leard lack of electrolytes will do this (in extreme cases, it's hyponatremia - the people in the medical tent after the marathon told me that).  I've also heard that dehydration/heat exaustion will do this (some nurses told me that).  If these will cause these symptoms frequently, then I should try to fix it that way first. 

Are those true?  Are there other causes that are just as common as those two?

Other notes: I weigh 145 pounds.  If I drink water on an empty stomach (while relaxing) it goes through me in 30 -45 minutes.  I seem to process it quickly compared to others.  I found that my gel packs DO contain sodium (55 mg) so since I had 6 on the run, maybe I had enough sodium... but I don't know how much is enough.  This only happens during high exertion.  Even my long runs don't seem to cause it unless I'm around 7:30 pace... this last one happened during a sub 7 minute pace for three miles, then a very steep 3/4 mile uphill. 

Anyway, thanks for the tips... meanwhile, I'm just monitoring it so if I begin to feel tingly (I always feel it lightly for a few miles before any drastic effect) I'll just slow down a bit, which is what I did this last time and my hands quit feeling tingly.

Mark