Yes I have the rim lock fitted, and I thought it was tight but maybe not! As for the nut on the valve stem I always tighten them as well.
Never had this problem before until I got this bike? I'm guessing all the torque is causing tyre slippage.
Never tighten the nut on the valve stem!!! Always leave it off or back it off until it hits the valve cap. By doing this you can see when the tire slips because the calve stem will be cocked to one side. Then simply let the air out, loosen the rim lock, break the bead, spin the tire and hit the brakes to spin it back the opposite direction.
Funny thing is my 450 hasn't spun the tire in a while but on my 125 I had to reset it last weekend. I guess it all depends on how good it bites into the rim and the condition of your rim lock.