Here is an article you might find interesting.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=28577[/URL" target="_blank">

At age 56 (I'm 57 now) I developed those symptoms. On some runs I did not have a problem. On others, I was unable to complete even the first mile. I never knew until I exerted myself whether it was going to be a good run or bad. About 4 months prior to noticing these symptoms I had a TIA (mini-stroke) that the doctors could find no cause for - I also had no risk factors. After the stroke there were lots of tests and exams done but no reason for the stroke was found. After the symptoms came I had more tests - still nothing. Finally, one day when I was gettin an echogram, I happened to be in fibrillation - it was not happening during any of the other tests and had gone unnoticed. They determined that I had intermittent attacks of atrial fibrillation and speculated that my stroke was caused by blood clotting in the heart while my heart was not beating strongly and then a clot loosened and went to the brain when the heart started beating regular again. The day they discovered the fibrillation they took it quite seriously and put me in the hospital for the day and shocked my back to normal rythym. Since then I have been on Coumidin to minimize clotting and Rythmol to help regulate heart rythym. No problems since.
My running history is serious running from age 30 - 45, 8 years no running at all, then about 1 year of running again before the symptoms appeared. I am 6'3". None of the doctors I have spoken to have ever mentioned a cause. They approved of my continued running. I hope I am not going to be on medication for the rest of my life but for now there doesn't seem to be an alternative.

What treatment are you using?

It sounds like you are constantly in fibrillation - is that right? My fibrillation was intermittent so sometimes I could go about running normally and sometimes it was impossible. I never was aware of any symptoms until I started each run. However, looking back, I remember being more tired and taking naps quite often. Again, I have had no problems since starting the medication.
I would like to get off all medication if I could because I don't like the possibility of long term effects from it. The Rythmol also costs nearly $400 per month. I have read about ablation but my doctor seemed to consider it only if the medicine did not work. Whatever treatment is used, I don't want side effects that limit my activity so since I seem to be doing OK on the Rythmol I probably should just resign myself to it.
I don't remember all the tests they gave me but I got a pretty thorough going over and all systems (heart, arteries, lungs) got high marks so I guess finding that out is worth something.
It seems strainge to me that this comes on so suddenly and for no apparent reason.
Hope your tests go well and you are able to get the heart beat under control.

Yes, I certainly would appreciate any information you get. I also am adopting a healthier lifestyle - mainly diet changes. I like the wait and see approach. If it does return, you'll recognize it right away for what it is and can take appropriate steps to correct it. I have not lessoned my physical activity - I have done 1 marathon since the diagnosis and have one scheduled in October. But like I said, my doctors never mentioned any connection to endurance exercise.
Keep me/us posted.