You may be hitting the rev limiter in fifth gear. If that's the case it wouldn't really feel like it wants to kill. It would just feel more like it lost power and it will wine and sound like it's bouncing between higher and lower pitches. For the throttle response problem...that is normal. I have an '05 YZ250F and it does that sometimes and has since it was new. Try adjusting the fuel screw on the bottom of the carb. That can help. If you are not sure which one is the fuel screw (since you don't have a manual) try looking online for a picture that will show you which one is the fuel screw so you know you are turning the right screw. Also, if the bike has after market parts like a pipe and silencer you may want to call the manufacturer of the parts, let them know what bike you have and all the aftermarket parts you have and they can give you the correct jetting settings for the carb. You may have to re-jet the carb. Hope this helps.