GM 3 wire O2 sensor dilemma

[ECMnut at aol.com]

I'm having a wierd problem with the 02 voltage in my GMC Syclone.
About 8 months ago, I noticed that the O2 numbers went from
950's to 8'40's... with no changes.  Fuel Pressure from my Walbro
pump has been check several times under boost, and it's good.
 I started making chips and adding fuel pressure
to get the #s back up to where it has always run the best (950's), 
but by the time I got to 915, the black smoke from the rear of the truck
convinced me I was *way* rich.  Now those numbers sound high, 
but I think Diacom does the wrong math with 02 calc, because I've
 had readings as high as 1020 on the 02 in the past and general 
concensus for these trucks is mid 900's is the place to be..
Besides I've owned this vehicle since new, and had a diacom as long,
and know where it traditionally runs the best..  I recently installed
a new AC/Delco 02 sensor, and it made no difference.   For
some unknown reason, my 02 readings are down more than 100mv.
I know that I could just say "screw it" and re-establish the 02 range
where the timeslips are good, but when working with boost, fueling
is not a good place to have a blind eye to the unexplained.
Could this be a bad ECM or ground somewhere?
Any ideas would be appreciated.
Thanks,
Mike V
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