Low O2 sensor readings

[Chief]

I hope yall can help with this one.  Doesn't everyone love a good mystery!

I have been getting a lot of 13 codes on my Monte (O2 sensor fault).  I
connect a DVM to it and it seems pretty normal, around .4V at cruise, .5 to
.6 V under acceleration, .3x V or so at idle.  However, at idle in gear,
when watching the O2 voltage on the DVM, using turn signals make it flip
flop around by about .15V or so.  Turn on lights, it drops near .30 or .2x,
turn on AC and blower motor and the O2 voltage drops to .1x volts!

This is wreaking havoc with the computer (CCC), I believe it is retarding
the timing completely after these codes keep setting.  If I clear them, as
soon as the system goes closed loop, the 13 code sets again.

I have tested the alternator, it's OK.  Even bolted in a brand new one, same
symptoms.  Had the battery tested, it's OK.  One thing is peculiar.  At idle
with everything on, voltage across the battery terminals reads 13.x V.
Good.  However, the guage on the dash reads more like 10 or 11 V.   I know
those things aren't perfect, but it reads way low.

Can anybody give me any suggestions on what the problem could be or how to
chase it down.  I really would appreciate it.  Surely this can't be normal. :-(

Thanks.

Ed Hilker aka "Chief"
84'SS - 700R4