OK need some help here. I have a 2002 C2. motor went bad (cracked head) so I was able to locate and purchase a good used replacement from a 2002 C4S, with a full Stainless exhaust. When I did the engine swap I did the normal maintenance stuff while the engine was out, Oil/water separator, oil cooler, plugs, new IMS, RMS and so on. I did order a new spec clutch and a lightened flywheel. Got everything in the car and running. Started right up ran great, but a day later developed rough idle and a miss, but only at idle. It runs perfect above idea. now the car did sit for a couple years before I purchased it so I'm suspecting bad gas. The gas gauge was showing about a 1/16 of a tank. I went ahead and put in 8 gals of good premium gas along with some octane boost and BG fuel Additive. I ran it again and at idea I have still have the misfire after about 20 miles of running it.
I went ahead and hooked my scanner up and found at idle I had active misfires from cylinders 1,2,3 when running live data. Then when I bring the car up from idle to about 2000 RPM's or so, all misfires drop off completely.
This is what I'm suspecting. At idea the crank sensor or other sensor is sending back reading to the ECU outside of the parameters, caused by the lightened flywheel at idle, causing the misfires. Could this be the case? Has anyone experienced this? Could the lightened flywheel at idle be causing this. If that's the case would a turn on a Dyno solve this? Any suggestion would be appreciated...
Again I'm only getting these misfires at idle, when I'm driving It's great. I have the live date to support this.

 

No, not that. More probable is an air leak that shows itself once the warm-up enrichment has tapered off.
Do a smoke test.

 

Or a bad fuel injector.

 

If you're able to read it out, I'd assume you'd be able to measure fuel trims too? I'd do that and see what they are. False air is typically spotted via high/rich trims, and leaky injectors via lower trims.
The work from there.