needed info on misfires

ok my svx had a bad coil pack (running rough and missing alot). i checked the codes found the cyl. and replace the pack. i got 3 used ones. after replace the svx ran great for 24 hours and as im sitting in drive idling my car jumps again with a small back fire. very minor compared to before so i figure maybe its a fouled plug in the same bad cyl. so i buy the cheap ngk single tips and pull old. the old look a little crispy but still good. they all looked the same. a little brownish no oil. so i figure maybe the used pack is bad i have 2 more so i put another used one in. so new plugs and diff pack is in and i start the car and it seems to run fine and normal for now. tiny shake due to exhuast but no jumps. what can cause this besides the packs and the plugs?????? if this happens again. im out of options.