Surprised that I've seen a couple of these threads recently. I would think its basic knowledge for fixed gears to know that if your cog is slipping, its just plain not tight enough. Isn't that a prerequisite to putting together a fixed gear? Don't get me wrong, I'm not calling anyone an idiot or anything because I've stripped a hub myself, but it was because I didnt have a chainwhip yet and the bike shop I took it too didnt tighten it for crap.

A new cog and lockring wont help you. Its rare that they are the ones with stripped threads unless theyre truly generic crap and sadly, if anything is stripped, its your hub.

If your hub is stripped, start with new hub, reputable brand cog and lockring and take it down to a good shop to have them install it. If you're going to be changing these parts out by yourself in the future by a chainwhip and lockring spanner, or a combo tool.