Rough idle/ stalls. Map sensor

Ok,  I've hit a wall on this one.  Haven't had a chevy stump me like this yet.  Customer
brought this in,  complaint was sudden loss of power.  Ran like a top,  then fell on its face.
Engine would start,  come up to high idle for about 3 seconds, then die.  Stepping on
throttle made no difference.  Fathering throttle after getting it to start would get it to run
very rough but timing advance was obviously off.

  General look over,  needed new plugs,  cap and rotor.  Wires we're recently changed,  and
all ohm'd good.  Made no change in running condition.  Dig a little deeper into dist,  found
ign mod had rusty bolts,  connector plugs corroded,  mag pick up rusted but seemed ok.  
Replaced ign mod,  no change.  Replaced dist assembly,  had to cut wire to try to set base
timing,  wouldn't idle well enough to set it proper.  Moved on to map sensor,  unplugged
vacuum from sensor and plug vacuum to tbI and idle stabilized...  Momentarily.  Unplug
elec connector from MAP and idle went to poo again.  I've changed out EVERY elec
sensor now with no change in drivability. I've pinned out the ECM voltages against the
FSM values and I'm close enough to them that I don't see an issue from the minimal
difference.  We now have THREE test ECM's and now a new HYPERTECH prom chip.  

So heres my question.  The only thing I do not understand is my ohm readings at A12
(system ground at ecm)  is 4.5+/- ohms key off,  and 62-78 ohms KOEO.  Why the
change?  Same for D1 at ecm, and at D2 (MAP ground)  I go from 5 ohm key off to 101.5
ohm key on.  These readings are from ecm to bat neg post.

Also,  my MAP sensor voltage (C11) is only 4.08v key on.  FSM says should be 4.8v.  I've
had 3 map sensors on it and they all read the same,  and engine reacts the same when
vacuum is applied.  Runs until vacuum reaches 12",  and voltage is about 1.4v,  then more
vacuum/less voltage and its sputters and dies.  Engine holds a VERY steady 16" vacuum
at idle.  

Help!!!!