2005 Avalance Low Oil Pressure on gauge after warming up

Hello folks,

I have an interesting problem that differs slightly from what I've seen
floating around the internet regarding this issue.


Vehicle info:  2005 Avalanche 1500 LT  4x4 with 5.3L Vin Z flex fuel engine,
but I run standard 87 octane.  I just purchased it recently and it has 175k
miles, with about 3000 hrs on the engine (averaging something around
50mph, mostly highway unless there is a glitch in the hour reading)

Issue:  Oil pressure gauge at about 45psi at cold idle, which drops to
around 25psi when warmed up, then after a few revs, or driving around, will
eventually drop to near 0psi at idle, and 15 psi when revving.  The engine
makes zero noise, it's actually one of the quiestest engines I've ever heard.  
Additionally, when the gauge is reading 0, there is no chime, red light, or
any low oil pressure, shut engine off warning on the information display.

What I've tried so far:
1)  Disconnecting Oil Pressure sensor:  Gauge stays at zero, but no
warning chime or low engine oil pressure, shut down in information section
of cluster which I find strange.
2)  Oil change:  The old oil was dirty but not the worst I've seen (I did not
change it last so not sure what type of oil was in there).  It had an AC Delco
filter.  I changed to 5W30 Mobil 1 synthetic and a purolator filter.  No
change in oil pressure.
3)  Oil pressure sensor:  I pulled the old one and put a new one on,
measured both with a multimeter and the old one had very high resistance
(about 400k ohms, yes K), the new one has about 200 ohms total, 135
ohms between 5V and signal, 65 ohms between low ref and signal, 200
ohms between 5V and low ref.  This did nothing, the gauge behaved the
same.
4)  Manual Oil Pressure Tester:  I hooked up an Engine Oil Pressure Tester
gauge at the sending unit port.  I had to use a stretch of about 5ft of nylon
hose used for manual gauges in a gauge kit, and there was a fixed larger
rubber style hose attached to my Harbor freight gauge, making for about
10ft total of hose, with the nylon hose restricting flow a bit.  Considering
there is no flow to the gauge once the fluid fills the entire space, figured this
would be fine.  The pressure was slow to rise with this setup but went up to
40-45psi cold idle which matched the gauge in the car, but only dropped do
about 34psi when warmed up (coolant at 200F according to scan tool, idled
for 15-20 minutes).  Clearly unless my setup is flawed, the oil pressure is
fine.  The engine sounds fine as well, it's so quiet I can hear the injectors
firing.  
5)  Wiring Tests:   I tested at the oil pressure sending unit for 5V with key
on, checked continuity from ground to a good vehicle ground, and then did
continuity checks all the way back to the PCM for all 3 wires and everything
checked fine.



Tech Review:  In this vehicle, the oil pressure is measured using a
potentiometer style oil pressure unit, and the PCM reads a voltage from it,
converts it to temp and sends it out on the data bus.  The cluster picks it up
off the bus then converts it to a reading on the needle.   What I do not know
is where the low engine oil pressure warning comes from.  Does the cluster
determine this simply from the oil pressure value on the bus, or does the
PCM send another message for it?  

In other posts for oil pressure problems, most folks get a chime and a low
oil pressure warning, please shut engine off.  They also often have an error
code reported and a service engine soon light.  I do not.

Currently, since my cluster has a few other minor problems (needles are a
bit notchy when they move indicating the stepper motors are fading) and
some lights are burned out, I'm ordering a dorman remanufactured cluster
for $190ish pre-programmed for my vehicle.

I do not have a GM Tech 2 tool to be able to read what the PCM interprets
oil pressure as being, which prevents my ability to troubleshoot out of my
own back yard.

Any advice would be appreciated.  I know I wrote a bible about it, but
maybe it will help someone else.  

Thank you.