The PCV valve in the valve cover. It's brand new and functional.

What's interesting is if I just stick the open hose inside the valve cover hole, the engine also starts missing and slowing down. Yes, there's a breather on the other valve cover. Removing it didn't change anything.

I always thought that there would be mostly fresh air in there... doesn't look smoky or smell gassy. Could it be that the new rings aren't seated yet, since they haven't been under load?