assuming that your crankcase has the correct gear set fitted for a 125 I would check that your timing is not too far retarded ,thats the likely suspect . I set mine at 19 deg currently . Check the points gap carefully (if you have them fitted )and then look at your fuel mixture at the middle to top rev band.(check spark plug ) Check your fuel flow rate from the tank, check throttle slide opening fully at full throttle and your air hose is not collapsing when you open the throttle up wide ...... all the usual stuff

if that is all spot on I would do a compression test .