Originally Posted by FastJake

I suspected a chainline issue as a derailed chain on a fixed gear is very dangerous and can cause lockups like that. But if you found your hub threads stipped, you've found your problem.

+1 Get the cog really tight then tighten the lockring.



Rotafixing works as well, but if you go too far you can break your chain or strip the hub. I've found JB Weld effective on a freewheel hub but then of course you can never remove the cog..