For O2 weirdness, I would check both the O2 sensor wiring and the ground for
it (Ground Point 18 or GP18).

For the sensor itself, there is a coaxial shield that should not touch the
hot wire at all.  You said you did all grounds.  Did you get GP18?  I just
solved a weird O2 related problem by cleaning and resecuring my GP18.  Its
hidden by the driver's side intake runners right where the head meets the
block.  It appears that this ground is responsible for most of the ground
signal for the O2 sensor.

HTH,

John Haddad