It does sound like the clutch. If you have a little free play and the cable isn't too tight, check to see where the arrow lines up with the line ( raised mark) on the case. Check also the return spring. If the mark doesn't line up with the arrow, it may be out of adjustment, you have to pull the right side case cover and adjust the clutch. Check then to see how worn they are, replace as needed. The bike weight and wind resistance and speed in 5th make sense too. With that mileage, it could need adjusted or if you have been hard on it, replaced. Keep us posted.