Originally Posted by explody pup
Is this usually due to a cheap hub or bad installment?
Yes.
It can be either. Some hubs will feature strip-away threads for your amusement, even if you do it all by the book. Misinstalling a cog can also F you up. Some cogs when properly installed still cause problems. We hates them, my precious.
The guy at the bike shop hand tightened it and said as I ride it, it will tighten and to adjust the holding ring (what's the correct terminology for that?) accordingly.
And did you? He's right, but basically it means you should go out, take a quick half-block sprint, then walk back and tighten down the lock ring. Others have suggested bracing the front against a wall and pedal against that to torque down the cog.
So now I'm drinking a beer, nursing my bruised crotch, and trying to figure out if there are any bike shops open on Sunday that would have a fixed hub.
Good luck with the beer and the beans.