I agree with Mr. Adams, I think the problem will not be with your sourdough culture but perhaps with just baking bread in general. The CO2 generated by the yeast will have a larger volume because of the lower pressure at altitude, making rising faster and over-proofing something to watch for. Flour (and everything else) may be drier at high altitude too, so liquids in recipes will have to be adjusted. Otherwise, the yeasts and bacteria in the sourdough should do just fine if you are doing fine.

Keep practicing is all I can say. I have come up with a few methods that are specific to my culture (I like it wet) but still produce variable results (I am also impatient). Keep baking and you'll come up with something that works for you and your culture and your conditions. And in the meantime, you'll have plenty of breadcrumbs from failed loaves for frying chicken, plenty of croutons for salads, and maybe even enough leftover less-than-perfect bread to start making kvass (my favorite recipe here, but using stale homemade sourdough bread instead of rye, he http://www.gourmantineblog.com/do-yo...-what-kvass-is ).

Let us know how it goes!