check the clutch cable out and make sure it working OK and adjusted proper
check chain sprocket is it loose at the output and chain for proper tension not too tight or loose
change the oil and do not try shifting fast also make sure the lever is proper mounted on the shift rod
page at grahams
http://grahamsmotorcycles.com/web/125_Gear_Shift.htmitems 7 am 19 do the shifting if they are worn out
they may be the problem... there are little pins on 7 if worn they can be a problem
19 can also be worn
clutch basket may need removed to see
crank nut may have reverse thread
do not remember it's been since 2004 since i been there doing any of this