Buy a chainwhip and a lockring wrench. If you're going to be riding fixed there's no reason not to own these tools. Then take everything off and reinstall the cog, and this time don't be shy about tightening it down. As the poster above suggested, take a ride (up a hill, if possible) with no backpedaling, skipping, or skidding, and then install the lockring. Again, don't be shy about tightening it down.