It is supposed to do that. It is warming up. After about 15 minutes of running it should stop, and idle at a constant.
let it run for about ten minutes or so befroe trying to adjust the idle.
The "Mixture" screw dosnt need to be messed with. I dont even remember where it is actually.
Just mess with the idle. you want it so that when its up, it will just barley start to think about turning that rear wheel.
As far as it dying at stop signs.... Could be a lot of things. This hasnt been ridden in a long time right? This might go away. 2 cycles like to be ridden, and run. YOu might need an oil change for the clutch to be rid of all water and other contaminants. It could be dirty crank oil.
Also, the engine will do this when its not warmed up. I would imagine if you drove for 20 minutes it would not do it anymore. Is that possible?