Hi. I have a different idea that might be worth looking into. This might sound strange... but I experienced something similar in the spring and early summer. I really thought I was getting asthma, especially since I also seemed to suddenly develop allergies (the two can often start at the same time). I have since realized that the allergies and breathing problems are probably completely unrelated. When I tried to exert myself too much I would hit this point where I just felt like I could not get enough oxygen. I would try to gulp back air as fast as I could, but my whole body just felt like it couldn't get enough. Do you have any other symptoms when this happens? I would get this feeling of breathlessness not just in my lungs, but in my leg muscles too (hard to describe). And my heart rate would be up at 197 bpm. And this was happening at a level of exertion that I didn't think was THAT intense. What I finally found out was that I was very iron deficient and very anemic. I always thought iron deficiency was no big deal really. But if you think about it, when you exercise your muscles rely on oxygen that is being delivered via red blood cells (with iron). So really, my muscles really couldn't get the amount of oxygen that they needed and so I was getting really out of breath... the bodies response when it needs more oxygen. This problem has gotten SO much better after a few months of iron supplementation. I can ride at levels that I used to be able to ride at. Does this sound like a possibility for you? Do you have any other symptoms of anemia? Just an idea.