What you have may not be a misfire.  At the speed range you
state, it may be transmission torque converter shudder
although that should be rare with the transmission updates
and a change to synthetic fluid for that vintage.  Have you
had the transmission serviced in cluding draining the
converter.  I suspect you may be well beyond the recommended
interval if you have not had this service done.

Another problem IIRC, may be a bad coil.  IIRC, your has the
coil-on-plug where there is an ignition coil on each spark
plug.  I believe there have been some problems with water
getting into the boots of some causing a short and misfires
on #3 or 4 cylinders.  Don't recall for sure but, I believe
the cure was a now coil and boot for that cylinder.  You may
want to check for a service bulletin on it.  If that doesn't
do it, you probably need a personal touch on it by a good
technician.  Keep in mind that something like this is hit
and miss on the diagnosis if it won't do it while the tech
is working with it.  Every once in a great while, you will
find the guy who had this exact problem a couple of times
and knows what to look for up front.

Lugnut

Then you have a fuel related problem.  The additive is either bringing your
fuel up to spec or compensating for some problem in the fuel system..  My
first plan would be to change where you buy your fuel.  Buy it from
somewhere with a high turnover and see how it goes after a few tanks.

Try a higher octane gas. If that works then the ignition timing may have to
be adjusted.

DID YOU LET THE OIL RUN DRY??!!