well, I have had my bike stall like that when I didnt have a good mixture going on and... when I downshifted the revs went way low and it stalled.  

I adjusted the screw a little and it was fine.  

I also have noticed this in humid weather in the mornings.

I've been noticing these kinds of concerns since early days of Bert's Savage forum in Germany. Please look at your carburetor when this happens and see if it is frosty on the outside. As the intake isn't heated I believe these bike are very susceptable the carb icing when the conditions are just right. Carb icing occurs when there is enough mositure in the air and the temperature of the metal in the carb is cold enough due to the pressure drop in the venturi. It does not freeze the fuel, only makes the venturi smaller and plug air and fuel bleeds. I had a 54 mercury sedan in college (late 60's) that did it regularly and the carb would be a big ball of ice, also had a early Chrysler K-car that did it when it was about 34 degrees out and snowing, no problem when raining.  It even had heated air intake and it tested OK, but would ice.  Let us know if you see ice pleae, thanks Max

You aren't happening to bump the kickstand with your boot heel, are you? That will instantly kill the motor.