Man, you've done alot so far. Forgive me if I missed something.

Did you check the O2 sensor with it idling at operating temperature instead of the bench test? From what I understand the flame bench test should not be used. It is obsolete now.

Also, did you set the idle at operating temperature? Mine idles high at first and then comes down as it heats up.

I had a bad ignition coil on mine not too long ago that would cause it to surge at high speeds. It was on one of the cylinders (#3 I think) that is not used at lower RPM's so it didn't effect the idle. Yours could be on one of the others so check the resistance per the service manual.