It does sound like the clutch is slipping. If it is adjustable, losening the cable may fix the problem. I'm guessing yours has a hydraulic clutch, though, which I am not that familiar with. It could be that the plunger is sticking in the clutch master cyclinder or slave cyclinder, or it may need a new clutch.
If you've ever thought about adding a slipper clutch or Rekluse clutch, this could be a good time to do it.