I'm working on a friend's '04 gt, manual transmission, BBK headers, off road exhaust, and that's about it for any modifications. It's got a real strange problem. It develops a miss while driving. Not just a small miss, but it damn near sounds like a jake brake. It happens under somewhat of a load, say cruising and going up a hill. Suddenly, like a switch has been turned, it goes into big rig mode and drops all it's cylinders, it runs but runs real rough. Pulling over and shutting the car down, and then restarting the car, the miss is gone. It will stay gone for a few blocks, or few weeks. But it never goes away for good. I've changed the plugs, on it, and pulled the codes, pretty much every cylinder is misfiring, but no lean cylinders. Anyone care to enlighten me as to where to search? The car runs great when this problem is in remission. Like I said, it's like a switch has been turned on and things go all haywire.