I was having this problem this Spring, and my doc, who tends to loathe prescription anything, asked me to track my diet for a month before she had me have an allergy test. I did, and tracked symptoms, and lo and behold, they were related. Certain foods really triggered bad breathing on long rides.
So she suggested I cut those foods for a month, which I did, and lo and behold, the asthma disappeared.
I also lost ten lbs. in the process and that probably helped my climbing.
Everyone's different but.. I'm glad she didn't just give me an inhaler script and call it a day.