Thread: 2001 cylinder 4 misfire idle only

Hi folks,
Total noob here. Saw several other misfire threads, but did not want to hijack, also did a search, but nothing exactly like this,so here is mine.
Have a 2001 GP with the 3800 series II motor, no supercharger, 165k miles.
Engine started misfiring month or so ago, at idle, after warmed up. In fact, engine idles fine even when warm for about the first 30 seconds after you start it. I suspect the computer runs fuel mixtures a little rich for that period and then goes to normal air/fuel ratio even when warm. This motor appears to run fine while driving, plenty of power and smooth, until you pull up to a light and idle, and then misfire starts.
Changed plugs and wires(they were due) and problem persists. Bought scan tool and shows cyl 4 misfire. swapped coil packs, still cyl 4. Sprayed entire top of motor with carb cleaner at idle and no indication of vaccum leak(thought I was gonna blow the place up though). Took to dealer and he said he did everything and also swapped injectors, still cyl 4 misfire. He wants to take off rear head, and I ask for my car back after $300 of trouble shooting. He also says he did a compression test and cylinder is fine. I am leaning toward injector as it does not look to me as if fuel rail was ever removed when I got it back from dealer. Also threw a fuel pressure gauge on it today and pressure is good at idle, and jumps up 10psi with vac line removed from regulator. This is actually my daughters car and I only get it for a couple hours every weekend to work on. I also put 32oz of Seafoam in a full tank of gas when I sent her home today. Might have forgotten a thing or two but that is most of it. Any ideas?
Thanks!

Hey FFDP, thanks for the response! The plugs were AC Delco and it was a custom Bosch wire set. I think the gap was .055, but that was a while ago. I did not swap wires as the exact same problem came up with all new equipment, so i figured I just didn't guess right.
One other thing I forgot to mention, when problem is happening at an idle, I think I can hear the motor go through 4 or 5 misfire cycles, it's pretty easy to hear it at the exhaust (maybe 1.5 seconds??)and then the motor will smooth out for for a few revolutions(maybe 4-5 seconds, I dunno), and then back to the misfire cycles in a pretty rhythmic, predictable cycle. Injector has to be firing at least some times, but maybe it take that long for enough fuel/air mix to accumulate in cylinder to fire it???
And by the way, I have never, ever, in my 25 years working on cars here and there, fixed anything with fuel injector cleaner(over the counter stuff). First time with Seafoam though, so we will see. Anybody have any luck with fuel injector cleaners?
Thanks.