This is great, I am so happy for this forum! I also felt very alone with this issue! 
I had this very similar symptoms also: I feel very dizzy and fainting and when that happens my heart beat is a bit slower (felt like low blood pressure) and when it passes it goes faster (felt like high BP), I got very scared the first times this happened, I knew it couldn´t be a pregnancy related thing (never been through pregnancy), so all other options were negative and ver frightening because all I could think of first was a heart related issue.
Then I start reading, also because the doctors I saw didn´t really knew what was going on. This was on and off since last Oct., there were months when I didn´t fell it at all and others when it was very hard: Never daily but maybe up to 5 fainting like spells and rapid beating in an afternoon (the worse one on Nov 9, my birthday :S ). I decided then to see a cardiologist; he says my heart is OK, and it´s being a vbictim of something else. He wanted to rule out a GI thing first and gave mi Omeoprazol and Metoprolol. Metoprolo has worked for my heart, now I do fell very dizzy but my heart is rather normal like (but I have gain weight and I can barely concentrate)
And that´s another thing, I feel tired all the time, sleepy and when I wake up I feel still very tired or with a slight headache that stays the whole day, I have recently find out that is not actually normal, but had that since I can remember is just that I feel now its worse, i really don´t feel like resting at all, and of course is very frustrating. I take iron and vitamis supplements, I´m not actually sure they work.
Then I went to an endocrinologist and turns out the only thing out of place is that I have very low cortisol level and that can cause dizziness, but I surf the internet very often and I can seem to find a relation between the fainting like and the sudden urge to pee or poop; for me is with both but and it is not every time, I mean, I there are entire days when I can go to the bathroom very normally an in schedule, but then during the late afternoon or midd morning I would have this heavy dizziness; and the worse is: it waked me up at night!!! I was never the kind of person who would wake up in the middle of the night (at least consiously) at all, and now I wake up sometimes with the urge to pee, then pee and then lay down and I start feeling that heavy dizzyness again (I imagine it happen a first time when I was sleeping and then I only feel this secuels)
I have been on cortisol only two days and really look forward any improvement... I don´t know what to do, if you have any ideas, suggestions or contribution I would gladly appreciate it (I will definetly try what the girl´s Dr. frien advice her to do, we all have stress and sometimes it just goes over the top) Thanks in advance for any kind of info. Hope we all feel better soon.