It seems to me it's caused by minor clutch drag and you should not worry about it. My bike behaves just like that as well. The engine has significantly less power at lower RPM and thus can't over come the drag. I don't think you will ever ride at such low RPMs and if the bike is moving there should be no problem at all. I think you will just have to learn to live with it since this little clutch drag is impossible to tune out. You can, however, change to different oil:
http://www.gasgasrider.org/html/gear...valuation.html