I'm not sure where to start but I bought a 91 mustang GT off of eBay a few months ago. Never had any issues until a week and a half ago. The car is fully built strocker 347 with a procharger only running 7lbs of boost. Edelbrock Victor Jr heads and intake and a dart block. The whole fuel system and cooling system have been upgraded. It has tremec transmission along with upgraded drivetrain and rear end. Now to problems. It ran absolutely beautiful for first couple months then I noticed it started missing around 3500rpm. I changed the plugs with the same ones n there autolite ap3923. The car ran so much better like a new car. This lasted for bout 2 months then turned to . I started checking most components in the ignition system. I thought I found the problem when I found no.7 spark plug wire boot was bad either from header or the steel mesh oil dip stick. I tried something generic and easy. I wrapped electrical tape around the boot and it ran great for the night but knew it wasn't going to last cuz I could smell the electrical tape melting. I ended up replacing the spark plug wire and went to the next hotter stage of plugs 3924 since the plugs didn't look like they were burning off the deposits. The cap and rotor are good and tried a new coil but to no avail. I tried a bunch of other things like changing fuel pressure, adjust timing. Everytime I make an adjustment it will run good temporarily like half hour or so then go back to crap like a sensor took over. Someone mentioned to the egr valve but it has been removed. The car has an AEM wideband which I know has to have a oxygen sensor. This is farther towards the end of the x-pipe. It appears there are 2 more oxygen sensor, one on each side closer to the headers but I don't know if they are hooked up. I just remember eliminated them in my first mustang. The car will run great first thing in the morning for about 5-10 minutes depending on driving then it will start missing at 3200rpm. You can drive the car normal as long as you shift by 3200. I kinda have come to the conclusion that it's the oxygen sensor. I was told that they don't do much til they warm up to 600 degrees. Which would seem right. I was also reading on here that it could be the tfi module which my dad thought it was n the ignition system but he doesn't know a lot about Ford's. I could greatly use any information cuz I spent most of my savings on this car. I knew I would have some minor things to do but this is mechanical strong and pulls like heck. If you need any more information from me please let me know. Thanks so much and greatly appreciated