The locknuts are tight, the wheel didn't move that I can see (no fresh scrapes in the paint).
By "cog slipping" do you mean that the lockring is not tight against the cog, so the cog can thread and unthread a small amount?
I agree that ACTUALLY skipping a tooth on the chain seems extremely unlikely, that's just what it kinda felt like.
I do have a master link but I'm pretty sure I installed it correctly (snapped together all the way).
Frame: Steel early 80's Schwinn World Tourist... very large, not sure the exact measurement but I'm 6'4" and have almost no standover clearance. Rear wheel assembled came from Sheldon Brown, the black ones, I could probably find the link if you need it, but it was a specific fixed wheel, not a conversion. BB is the one he lists as "the perfect compliment to <whatever> crankset" on his fixie cranks page. The cranks are BulletProof BMX(I think) cranks.
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