Originally Posted by cny-bikeman

First, chain "tension" and length, derailleurs and shift levers have nothing to do with the problem. Your problem is very unusual in that typically with a new chain there will be one or two cogs that will skip and the rest are OK, not the reverse. The only scenario that makes sense to me is that the cog was actually not new but rather used and cleaned up and soemone installed a new 15th cog.