Oddness

I have had this. It was the cog not being on tight enough so it would slip when starting off and then again (as i was riding without front brake) if i stopped hard.

the solution to my problem was lock ring off and tighten the cog up good and tight and then the lockring back on nice and tight.

+1

Go the whole hog, remove the sprocket completely, clean the threads, apply copper grease, sprocket back on tight, lockring back on tight.