I just had this problem for the first time on the weekend....
I noticed it just outside of Sudbury, When were trail riding at a bit slower speed. It seemed worse if I got on the brakes comin in the corner (almost like it wasn't back shifting). I would give back in the gas, it would bog, then I would give it another shot and it would slowly come back. Once we were riding abit faster (ie, faster corners) It didn't seem to happen.... I have 2195 miles on mine now, have been running 87 in it since day one... and it's a summer build 600. Any ideas?