Benjamin Gough

I have a 1999 z71 5.3l 246,xxx miles and can't get rid of a missfire out cylinder 7. About 2 months ago I had multiple missfire code, that made my truck hesitate and have a rough idle. I changed all the spark plugs, wires, and coil packs during that and it did not help, so after that I put some seafoam in the vaccum line. after doing that my missfire ceased in a short amount of time. Now about 3 weeks ago I had ran out of gas and rolled the truck no more than 1/4 mile to get it off the road. after putting gas in the tank and driving off the truck died, so I had wiggled the battery cable and it started back up. The truck went all day perfectly fine then coming it I got an CEL. when I hooked my scanner up I got yet again missfire cylinder 7 as well as "catalyst system effiency below threshold 2". I had went ahead and put some cataclean gas treatment in and didn't seem to help, later that week I went ahead and seafoamed injectors again thinking they sucked up to much gunk at the bottom of the tank but that did not seem to help. i also changed the fuel filter to see if it had maybe gotten gunked up but still having a missfire. it is mainly missfiring while idling or during low rpms, rarely will it missfire when Im going about 55 but anytime about 45mph and below its missfiring. not sure if this makes aNY difference but I have an aftermarket exhaust and when the engine is missfiring the exhaust does not make the constant noise it normally does, its inconsistent and goes putt putt putt . Any ideas