Or criss crossed coil pack wires

 

Or this. Good call, Trav.

 

if you have a stock ecu, it may will be adapting to the exhaust changes, give it some time. If not I would clean/replace 02 sensors, I would clean my cam and crank position sensors, I would clean my maf sensor, I would check for good leak free connections on my intake, I would pull my spark plugs and check for any sign of an oil leak. While you baby the car, you never know with a z how it has been treated before you, I once bought a z, luckily for a drift missile, and they claimed it was a one owner, stock kept, babied car. Once I really started getting into the car there was a 6 puck clutch, the differential wasn’t even the stock gearing, and the ecu had a very bad tune on it from what I would assume the guy got off of ebay. I payed over for the car because it’s rare to find one that’s been treated well, and I really wish he would have just been honest because I still would have bought the car it wasn’t very clean, but luckily for me there were no real issues from what I would assume from the car being drag raced. If those simple resolutions don’t work out, you could have some bigger issues like a small head gasket leak. Hope this helps! Good luck!

 

did you ever find a solution to the problem? I'm having that in my 06 350z enthusiast VQ35DE, it only happened at idle of course and it seems to oddly be worse when the AC is on. I know you posted this in 2009 but if you ever figured it out it would be greatly appreciated if you told me what it was.

 

no clue if you're still active. but I might as well ask. I'm having the same issue. it seems it only misfires on idle and when entering deacell right as I let off of the gas, also when turning on the AC when rpms raise to 950-1000 it seems to stop but is still very slightly noticeable. All the coil packs are new, spark plugs are new, fuel system is new, and passes compression. the engine has 130k ish miles and never throws a p0300