Well, it's not impossible. Certainly sounds like the clutch is slipping, but it could just be poor adjustment too.
Easy way to tell is to go out and sit at 30mph in 5th gear, then floor the throttle. If the revs rise but the speed doesn't then yes, it's goosed.
However, are you referring to upshifts or downshifts? I wonder if it's anything to do with the rev matching stuff...