Engine runs funny and upper end

I am helping my friend who has a grubee 66cc 2 stroke. When exelerating it runs smooth and fine up untill the rpm's get up past about 2/3rds throttle. At that point the engine vibrates reall hard and the engine sputters or hesitates a little. The engine kit is pretty new with about 10 miles on it. The strange thing is it didn't do this the first day. I think he might have messed with the needle valve adjustment on the carb. Maybe it is running a little lean? But I don't think that would explain the vibration. The vibration is wierd, the engine runs very smooth up until 2/3rds full speed then it is like a switch is turned on and the vibration starts imediately and is very bad. It almost feels like my butt is going to vibrate right off the seat! But as soon as I back off of the the throttle and just cruise at about 20-25mph it goes away. Any Ideas? Thank you.

good idea. i just double checked and every bolt and nut on the bike and engine are tight.