I also have had migraines for a long time, as well as dizziness in recent years. One of the things I have learned is that some form of dizziness is EXTREMELY common in those with migraine. Here is more information about this:
http://emedicine.medscape.com/article/1159385-overview
If you google something like 'migraines dizziness', you will find many, many scholarly articles about this, so it is odd that your doctors have not mentioned it. In fact, in reading about this, you will also learn that among migraineurs (people with migraines), Miniere's and vertigo are also frequently found.
I suggest you look for a neurologist with a special interest in migraines--my experience has been that the problem becomes worse with time, so both understanding and treatment are very important.
You may want to check out Dysautonomia. How is your blood pressure? If it is typically low it may be causing your symptoms and simply adding some salt to your diet may help clear things up but make sure you discuss this with a doctor to be clear that you have low bp because too much sodium isn't good for you either. Take care. I hope you can get to the bottom of this soon.
There is nothing in your post that calls out heart rhythm problem in my mind. Yes, a low heart rate and/or low blood pressure can cause dizziness, but you didn't mention any heart issues. There may be better forums to place your post.