Tim, you said this happens in the "middle cogs." By chance are your friction shifters loose and move more freely in the mid range? If so the spring in the N/R RD might just be pulling the chain off to the next cog. Does it tend to only skip in one direction? Down to a smaller cog? The other possibility is that the RD is bent and the jockey pulleys not lining up with the freewheel cogs correctly.