Have an update. I had been using the crappy clamp-on WBO2 bung, now it’s gone. I removed my header, welded in a bung, did a pressure test on the header, absolutely perfectly air right. Put on new aluminum crush gaskets (sorry if that’s not the right term), they're the soft aluminum ones. I put a new one on the cylinder head and the collector. I ran the car, it’s better, but still getting a small variation at idle. What the heck else can it be? (Also, I reset my load on the ECU, so I have a fresh base calibration.)