I wonder if perhaps your power loss has nothing whatsoever to do with altitude, but instead is caused by carburator icing?
Normally, a change of elevation from sea level to 1,500 feet should result in no detectable difference for a carburetted engine. Heck, if it did make a difference, no-one would be able to drive in Switzerland (or, for that matter, across North America).
At this time of year, outside air temperatures are rather low (perhaps 10 to 15 degrees C) and the air can be quite moist. When cool air is drawn into the venturi of the carburetor, the temperature drops, and ice can form in the carburetor throat. I had this happen to me on several occasions on a VW Beetle. The result is a slow but steady loss of power, and eventually, the engine just sputters out. It might just be coincidental that you are travelling uphill (rising in elevation) as this is happening.
Next time you encounter your 'power loss', see if things return to normal after the engine has been turned off for about 60 to 90 minutes (sufficient time for the residual heat in the engine to melt any ice that may have built up in the carb). In particular, if you have the chance, park the bike in a heated enclosure for a few hours, then start it up and see how it runs. If the problem disappears after either of these two actions, then your problem is carburator ice and not mixture.
This Wikipedia article explains the phenomenon: Carburetor Icing
, and this aviation-specific article provides considerably more detail: Carburetor Icing: Part II, Symptoms and Remedies
Note the reference to "temperatures between +5C and +15°C" in the aviation article.