Its pretty hard to say. It could be the external shift mechanism under the right side case cover, there could be wear on the star on the back side of the shift drum, or on the fingers of the mechanism, or it could be an internal problem in the transmission, such as a worn shift fork. It could possibly be somthing simpler as well, like out of adjustment on the clutch screw, or a loose foot shifter.
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