Hi, I'm new here. My name is Scott and I drive a 1997 4.0 Wrangler Sport with 136,000 miles. Lately I've been having this problem that is completely random. Just for a little background info: My header cracked and my pre-cat O2 sensor was bad. I replaced the header and the O2 sensor but not before the header cracked all the way off and dropped my exhaust into the front drive shaft, which left a hole in the exhaust. I had just bought the car not but a month before. I replaced the exhaust and now it starts to stumble under load. It is bucking and misfiring but only when it is in gear and I'm giving it gas. If I coast in gear it doesn't do it and if I just free rev the motor it doesn't do it either. It started doing it right before I fixed the hole in my exhaust and replaced the O2 sensor but after I replaced the header. At first I thought it was a burnt/warped valve but it comes and goes. It only seems to be doing it when the car is warm and it's just to intermittent and random to be a valve. Then I watched my tach move around while it's missing so I thought the coil might be bad. So in went a new coil, new rotor and cap, and new plugs and wires. And it's still doing it. The exhaust is in fine shape now, the CEL is off and the only codes present are 12 and 55(12-battery has been disconected recently and 55-end of codes). It is completely random. It will stumble for a minute and then stop for 3 minutes, then stumble 2 or 3 times then stop for about 5 minutes. Any ideas or suggestions are greatly appreciated!