Try removing the cog and lockring and check for stripped threads and make sure everything is wrenched down tight when you put it back together, like Retem said, sound like it needs some tightening. Just start poking around and see what you find.
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Ode to the after work nap ( ride your bike instead)
Ode to the nap
The evil, evil nap
It lures
you succumb
But only with good intent
Shortly I will rise
But you do not.
Do not succumb
To the evil, evil nap