condition of the wiring harness going to the O2 sensors. I had some idle weirdness going on years ago. Despite the heated O2 test passing, it appeared that the ECU was not getting a great signal. Turns out that from the O2 connectors on the EFI harness, the wires were pretty corroded about 16" up. I ended up cutting and splicing in new wires. After I did that, the weirdness went away.