My '28SP had been stood for a while when I bought it and for the first few miles it did something similar. It would reach somewhere between 7,000 and 9,000 rpm (can't remember exactly) and would then sometimes run badly / hesitate for a bit before taking off again cleanly. In my case I think it was sticky power valves due to lack of use. After a 20 miles or so it seemed to clear up itself and it has not happened again.

There's a lot of experienced NSR owners on here so I'm sure someone will offer some more informed advice shortly .