So I had a similar problem except that I am fairly certain that there isn't a compatability problem between my lockring and cog.
Yesterday, I was sprinting across center city from a stop trying to get my speed up and i got out of the saddle to accelerate faster. My cog slipped. My original thought, as the cog wasn't freewheeling it stopped after about a 1/4 revolution, was that i leaned forward and caused the wheel to lose traction. I pulled off to the side, examined the bike. Everything seemed to be in order. Then i was on my way. When i applied back pressure to the pedals approaching the next light, the cog slipped back a bit (less than the original forward slip but still substantial).
Here is why i believe my case to be different:
I just bought a surly hub/mavic rim wheel from my lbs. However, my cog and lockring came stock on the 2004 Fuji Track. So the likelihood that the cog and lockring are incompatible is not very good.
This leaves two realistic options to me (because i don't think i stripped anything):
1. Is it possible that there is some minor incompatibility between my cog and hub that needs a spacer type thing
2. My legs are stronger than the LBS mechanic that transferred my cog from my old wheel.
thoughts?