I mounted new-old wheels with a 6spd Shimano freewheel recently bought on Ebay on my wife's Bianchi and now the chain is skipping badly on the 2nd smallest cog, which has teeth that are more "twisted" than the larger cogs. The freewheel looks to be in like-new condition. The bike is a hodgepodge of parts with Ultegra 8spd barcons ratcheting in friction mode, 8spd chain, and Veloce 10spd CT crank.

I'm wondering if some combination of the chain being too narrow, the cog's teeth being effectively too wide, and the ratcheting being too coarse prevents it from running smoothly.

Thanks for any ideas.