I've had some pretty bad slips as well, lads at work have dealt with loads of alfine and nexus as we're a gazelle dealer, they reckon it's the shifter as it's not as accurate / well indexed as a rapid fire is and the hubs are very sensitive to cable pull..:
Not that I'd attributed to the shifter, I've had a few grindy/clunky moments but put it all down to the Nexus not being quite as good as the Alfine.
You think it might be the shifter?