I take off at a stop light today, and my pedal drops down as if the cog was sliding on the hub. At the next light, when i was slowing down, my pedals over-rotated again, now in the other direction. I freaked out. First time I thought it was a skid, but the second time I knew it wasn't.

I was afraid my threads were stripped. Luckily, they weren't. My lockring had loosened up, and the cog was screwing and unscrewing against it.

The moral of the story: check your lockring frequently and make sure it's TIGHT. Especially if you ride brakeless.

You mean on a reverse threaded lockring?

what other kind is there?
if you use a same-threaded lockring you better run some sturdy brakes.

The reason I asked is because as you probably know, alot of converted fixies use old threaded freewheel hubs and use bottom bracket lockrings to secure the cog which is obviously threaded the same way as the cog. In some instances if these lockrings are not locktighted, cog and ring can spin off...

just checked...my **** is tight!