Just had the same problem 2 week ago with my Gas Gas. Took the water pump off and removed the seal and impeller shaft, the seal didn't look bad at first glance but then I noticed a tear. Also the impeller shaft had a nice groove it from the seal. Replaced them both along with the O ring, flushed the oil and coolant about 3 times, running the bike for about 5 minutes between flushes just to be sure I got it all cleaned out. I've ridden 3 times since without issue. Not sure if that helps much but hope it does.