I had the same symptoms this past spring. 2nd bird & have ridden these bikes for 10 yrs. Anyways, it WAS the slave cylinder seal as mentioned above. I ended up just buying a used slave cyl since I got a great deal ($25 !). I flushed the entire clutch system and it's been perfect.
Moral of story... change all fluids at least every other year (brakes & clutch fluid).