It is most likely a clogged EGR port on that cylinder. Instead of one big EGR intake port they have gone to 6 small ports. One for each cylinder. If one clogs the PCM interprets it as a misfire. It is pretty easy to fix, remove the upper plenum and clean each port with carb spray.

 

Update: I'm pleased, but a little embarrassed to say it appears it was in fact the PCV hose. This was forum member MJP68's first suggestion. With all the plumbing under the hood I mistook it for another hose. I should really spend the money and invest in a repair manual! The public library version was 1997. The PCV hose was collapsed where it enters the manifold and it had a noticeable leak when I touched it (hissing). The rough idling is now gone so I'm keeping my fingers crossed. I tried to take advantage of the PCV hose recall in 2004, but unbenounced to me it was replaced in 05 during other service. Since it was replaced in 05 I'm wondering if I might have done something to cause the hose to break while replacing the plugs on the rear bank 4 months ago. mox nix.

Side note: Called Mazda Parts and was quoted $28 for the hose. When I went to pick it up they said it was $68. They said the $28 hose is the old recall one. Why they are selling the old recalled hose is a mystery, and why the new hose istriple the cost is also a puzzle.

Thank you all.