alright, well lets begin

-No knock
-o2's reading relatively normal
-1 plug wire on passenger side looks minorly burnt around the boot
-all spark plugs changed
-backfires badly when idling
-only happens at operating temperature and progressively more noticeable as drive time increases
-only mod in last couple months has been longtubes a week ago

im at a loss...could the 1 plug wire be arcing once it warms up or something?