I realize this is an old thread but thought i would update on this issue. Even the mechanics couldn't figure out what was causing the problem and they said it would be pretty expensive to eliminate it via trial and error. So based on the symptoms i was getting and taking an educated guess, I went ahead and asked the mechanic to just replace the fuel pump (back when i had started this thread) and i'm happy to say that over 3 years later the issue has not reoccurred. Looks like that was the culprit.