Derbi Senda DB50B0 Clucth or Gearbox problem

[DerbiSenda70]

I have a derbi senda which i took to get fixed in Gloucester to have a 70cc Airsal kit fitted with 17.5mm carb and 95 jet and my gearbox sort sorted out as it was playing up would be all fine outside the powerband but when i go to 5th or 6th the the powerband the revs would fly up :S so they rang me few days later saying its all done kit fitted running fine but they could not find no gearbox problem when im pretty sure there was tbh so im running in the bike at the moment done 50 miles now been boring but good to be back on the road and i can feel the revs going slightly higher when going from 5th to 6th again but in not in the powerband my clutch is jerking abit when i pull off so people have told be replace you clutch plates which i have just ordered hopefully that will be it sorted but a few people are saying its the gearbox i dont know what to think really ?

[P.]

Worn clutch plates/clutch slipping