I currently have an 88' Mustang 5.0 Lx. I've had this misfiring issue but it progressed. I don't really understand if it is progressing because it is simply cold outside (17 degrees) here in NJ or if its because of an idling issue or what. I've included a video below showing the issue (you might have to turn up your volume) and You can actually hear the misfiring/popping sound. A little bit of information: My oil pressure psi was about 47 psi, my water temperature was at about 180(its usually between 180-195, not really sure what it is suppose to be around). My gas was full and my battery was normal it was at the 13v mark I believe(its the longest "-" about 1/4th of the way up from the bottom). I bought the car back in November from an older guy who took pretty good care of it but when I called him he "didnt know anything about it" of course. The car is completely stock besides the BBK Cold Air Intake, short throw shifter, BBK headers, Mac x pipe and I believe mac mufflers. The previous owner said also that the engine had only 100k miles but it just rolled over again and I personally believe it is on 200k but theres no way to tell. I've replaced the Egr valve and position sensor, the fuel filter, the sparkplugs with motorcraft coppers, replaced the igniton wires with auto lights, replaced the distributor cap and rotor with msd and even did a 3g alternator 130 amp swap with the bigger power wire that connects from the alternator to the ignition system. I went to clean out the Idle Air Control valve and it looks almost brand new and didnt have any buildup although i still sprayed it with carb cleaner. I have the speed density system not MAF and i currently have a MAP sensor. The car was in 3rd gear in the video, I was doing 35mph at about 1700rpm and It started misfiring/popping. It used to only do this with the headlights on but now its most of the time, at least once a day. Please ignore my stupid talking toward the end of the video. I included two videos, one while driving and another one with my idle moving around while i was at a stop light.
Thank you for your input.