The spark plug heat range sounds good too. That kind of leads me to think your throttle blades may be too far into the transition circuit?
I have found that at higher altitude, most carbs want a larger idle restrictor because there is less air pressure above the fuel, and the engine will also have less vacuum. I have never had an engine at this altitude (6,000 ft) that needed anything higher than a 6.5 power valve, usually I am using 3.5 to 5.5 power valves.
If you have a rear power valve, check it too to make sure fuel is not flowing past a blown power valve.