1) remove the cog and lockring; inspect the threads of all three (hub, l.r., cog). you'll know pretty quickly if the threads have been shifting or are stripped.
2) if no stripping has occurred, either take the wheel to a shop to have it installed properly OR install the cog properly by first applying grease to the hub's threads (no need to over do it); using a chain whip to apply adequate pressure; apply grease to the lockring portion of the hub's threads and use a lockring wrench to get it flush with the cog; apply a second push to the chainwhip/cog, and immediately chase that with pressure to the lockring. this should be sufficient; if you cannot tighten the cog (first) or the lockring (second) any further, you're good to go. get out and ride a mile, then re-tighten. this second part is often overkill, but will let you make sure you didn't screw up on part 1.
3) rocking suicide fixed (hub on a freewheel side) isn't adviseable, as the mechanism you use to keep the cog in place when subjected to skids isn't going to be nearly as strong as a reverse threaded lockring.