my 04 gtp compG (no mods) is reporting a misfire on the obdII can anyone help?

Maybe this will help.
http://www.obdii.com/codes.html
http://img.photobucket.com/albums/v425/kustomkid54/santaanim250.gif

yeah I have reader and it reads p0300 but why is it misfireing? And why dont i notice any performance decrease?