When moving to altitude rule of thumb is you want to drop jets one size for every 2000', you may need to reduce your power valve one size as well (i.e. from 10.5 to 8.5)... what do your plugs look like (rich/lean)? What rpm does it idle when running cold? How is your timing?

Does the exhaust smell exceptionally rich? Do the insides of the tailpipes have any color to them? I think you are running too rich with a carb that is tuned for air... you have less air so you need less gas.

-Chris