The high voltage path runs from one plug, through the coil connector, through the coil, through the wire, and to the other spark plug. So a bad plug connector can indeed affect spark quality on both plugs. The coil connectors only last so long. Folks who have been hanging around here long enough have learned to replace them every other set of plugs. Suggest you give it a shot. Shoot, they're not much more than 10 bucks each and only require 10 minutes to install.

If that doesn't fix it, then you've likely got a fuel problem on #5.