I am surprised no one has repsonded to you yet. Normally they jump all over questions like this.

1. Did you make ANY changes just prior to the first occurrence? (always go where the hands were last)
2. What kind of ignition system do you have?
3. Does it run crappy when you try to increase throttle or does it idle crappy too?
4. Have you inspected the plugs? How do they look? (rather, how did they look when you removed them?)

Sounds to me like maybe you arent getting any vac advance or maybe your carb went out of tune for one reason or another. I think reading your plugs will tell you alot and I dont believe that it is weather related. I am far from an expert on these matters though. Sorry I dont have your answer but maybe my comments will spark something for you. Let us know how it works out.

btw: I also have a street avenger, brand new, havent fired the engine yet. Hope that is not your problem.