i don't know if it's coming off the cog or chainring.
my chainring bolts are loc-tited in and my frame is ok, it's three months old and was cutsom build to minmize torsional twisting in the BB.
the entire drivetrain is 1/8". sugino 75 chainring, EAI cog, and a cheap KMC Z chain, which isn't very old, but suspect is the culprit.
thanks for the tip on shining the light behind the ring, visitordesign. i'll try that this afternoon and see what i find.