For a very long time i had been dealing with a shudder/miss right around 4000rpm. At the time i was pretty happy with my street tune:MAF,torque and VT. Was getting ready to VE tune then send it to a dyno to polish it. To make a long story short. My PC crashed, my tooner lost my base tune file and then passed away (not Mowtown GRHS). The miss started at that exact time though. That being said, I had to assume that it was something in the tune that either I did or he did, or a combination of both. Spent months going over the only tune I had trying to figure out where the tune went wrong. Picked over stock files to try to somehow rebuild a stock base tune to start from, since the one i had had some kind of flaw. The miss was like clockwork, so I had, again to assume there was some kind of boundary/value/breakpoint/fuel pressures that was out of whack. Plugs looked fine, checked injector resistance...good.
Plug wires....
One was just barely out of spec. It was not arcing to ground or anything, but had broken down just enough to cause it. Car only has 21k on it so I'm guessing it might have been from the factory and the headers might have helped it along. The other 7 were spot on with one another. Bought some taylors and for kicks just replaced the one suspect....and all was well. Kept telling myself to take it somewhere and scope it but again, with the timing of when it happened i thought it was something I directly did.
Just glad its fixed now...
Thing is had it been someone else's, that would be the first thing I would check