Does the problem happen after an hour or two of cool down or are we talking several days? When you try a cold start does it catch and run for a second and then quit, or does it just crank with no signs of life?
As an ex-Renoite, knowing you're at almost 5000 feet elevation and it gets pretty cold there, make sure the choke plate closes all the way when the choke knob is pulled. Look down into the carb before you cold start it next time and give the throttle a couple of quick "gooses", you should see (and smell) streams of fresh fuel squirting into the venturi.
During cold weather, most 235's are going to require a few pumps of the pedal and full choke to get going.
