I had a problem with the bottom headstem bearing on the WR250, it was full of mud and dirt. The race looks good and so do the bearings, I think I am lucky that I check them about every second oil change, so every 1000kms. The bearing seal looks fine, but this was obviously the problem. I have a sneaky suspicion that it happened about two rides ago as the rain has started to come, and the puddles have to be ridden through. I really noticed the problem when I put the new suspension on, it didn't want to turn at all when riding. I first thought it was just the new suspension just bedding in, but when I put the bike on the stand, turning the bars took some effort. I also suggest you check your bearing right away if you have a new bike and haven't greased them yet, Yamaha doesn't like grease for some reason.