2008 altima coupe 2.5 misfire

I have an 08 Altima that has an
occasional misfire... Not when it's cold,
but after it's warmed up and running
for a while when I slow down and
throttle to get speed back up you can
feel it misfire maybe 2 or 3 times. At a
stoplight you can feel it misfire maybe
every 5 to 10 seconds but if I throttle it
hard it won't do it. I replaced plugs
three were looked really bad one was
shiny like almost like it was new
replacement of the plugs improved the
performance but did not fix it
completely. It is also not showing a
check engine light, will it still show a
code if I put a scanner on it or what
else could the problem be?