My 5.0 runs perfectly when its completely cold, and also when it is up to temp, but about halfway through the warm-up period, it will misfire for maybe a minute or so and then go back to running great. I can tell because the car shakes at idle and it feels down on power during acceleration. Sometimes it does not miss at all, I would say about 80% of warm-up cycles it misfires. No check engine light. This has been happening for a while and I haven't really concerned myself with it too much because the car runs perfect 99% of the time. The motor is all stock except for EGR delete, long-tube headers and h-pipe, and an air intake. Does the car need to be tuned for those modifications? What could be causing this?