I have a few thoughts on possible things that you might want to have ruled out. I'm not a doctor, so they may all be wrong, but I can point you in the direction of some reading material to see if any of this "clicks" for you.
One of the things I thought of immediately was mastocytosis or MCAD. This can manifest in various ways, and different patients have differing constellations of the symptoms ... hopefully you can get the gist of that from reading these links:
http://www.nytimes.com/2009/06/07/magazine/07wwln-diagnosis-t.html
http://www.tmsforacure.org/mastocytosis.shtml
I think defecation syncope is unlikely, particularly if as you say, the fainting is often preceding the bowel movements.
http://www.medterms.com/script/main/art.asp?articlekey=9312
It sounds like you've had some cardiology workup, but not a full workup for the syncope/pre-syncope (fainting/almost-fainting). A tilt table test might give more insight into this. It could reveal a number of different causes for your dizziness, fatigue, and fainting (sometimes GI sometimes are even related). These disorders (including vasovagal syncope) are under the umbrella of "dysautonomia" and you may want to post in the dysautonomia forum here on MedHelp to find out more about testing for autonomic disorders as well as the types of conditions that could possibly cause the symptoms you're describing. Here's the link to our forum if you're interested in posting over there as well:
http://www.medhelp.org/forums/Dysautonomia-Autonomic-Dysfunction/show/266
Here's an article that's often helpful in helping people understand the difference between cardiac and autonomic causes of fainting:
http://knol.google.com/k/syncope-fainting#
I hope you get some answers soon!
-Heiferly.