How many watts are you putting into the Nuvinci? What kind of speeds are you hitting?
...Could you check the hub temp next time you ride for around 10 miles?
I have temporarily dialed down the power to stop the chain slipping, so it is now only about 2.8kw, and gets to about 32mph max on the flat.
NV hub probably gets about 5 deg C above ambient after 3 or 4 miles.