Linnie,
You may be exercising more strenuously than I do but, I also have time lapses. The episodes don't always correlate to exercise. I also have been extensively tested and told all is normal. I have been put on nighttime oxygen which only raises my percent oxygen by a couple of points. I am a month from my 81st year and am told by my nurse friends that I "do not present as an 80 year old". I have been told this often enough that I am beginning to believe it. Maybe like you, who has kept healthy and active, we aren't taken seriously when we know something is wrong as we present differently from average folks our age. I hope you find an answer. If you do, I hope you share it. I find myself slipping into a funk over this because no one thinks I'm ill.