Does it have any mods (pipe, airbox, open air filter?) Is it running rich, or lean? Take it out, and hold it wide open in a high gear. Kill the engine when you are in the high gear. Take out the spark plug. Is the color on the plug black or white? Black = rich, too much gas, and White = lean, too much air and HOT! The jetting could be off. Is the choke on off after it is warmed up? What happens if you turn it on while riding? Check the numbers on the jets, make sure the needle clip is correct, and refer to the service manual to get an idea where you are at.
The other thing that comes to mind are a loose wire, faulty coil, stator, or cdi. My first guess electrical wise would be the coil. Make sure it is not cracked, and that the wire to spark plug boot is plugged in all the way, and not damaged. Its hard to diagnose, it could be electrical, but it sounds more like a fuel issue to me. Good luck!
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes