Just a thought to add to what Slacker said about gaz cans. These tanks are plastic, and I magine it might be that on the hot dry days we have here in the high desert. That there might even be condensation occurring in the tank itself.
I had experienced symptoms similar, in climbing a steep grade and not having a lot of air flow around the radiator the machine stalled and back fired, but would idle when started, just didn't like any throttle at that point.
Also as I mentioned in another thread, this occurred on flat ground as well previous to this ride Saturday. So I was thinking a vapor lock thing? But this makes me wonder about condensation in the fuel?