Well, the Impala had been sitting nearly 4 months.
After thoroughly checking for different things that could cause the problem I got tired of seeing it sit there. I made up my mind to have it towed to a Chevy dealership for them to figure it out.
Taking it to a dealership is always an absolute last resort as I just dont trust they will be truthful with me when they diagnose it.
Anyhow, I decided to to one last check of all the fuses and probe them for voltage.
Doing this I found the problem.
I probed the fuse at the panel inside the Impala labeled "pcm/bcm/clstr" and found it to be reading much less than the battery voltage.
I was actually using a probe light, and when I probed that fuse the light on my probe lit very dim.
Every fuse on the fuse box panel lit bright, but the "pcm/bcm/clstr" did not.
Turning the key on the ignition switch on and off several times, I found that the circuit I was probing would light up proper (12 volts) in some instances.
When that circuit would read 12 volts the car would start right up.
Leaving the test probe in the circuit, the car would shut off after a few seconds and the circuit would simutaneously drop below 12 volts.
I then did a google search and found out that if the "pcm/bcm/clstr" circuit was reading below battery voltage, the probable cause would be a bad ignition switch.
I did'nt change the ignition switch however.
Instead I ran a wire from a good 12 volt source thats holds current only with the key positioned at on, and ran that wire to the "pcm/bcm/clstr" circuit, and presto problem solved.
My Impala is up and running with no problem.
I would never have thought that the ignition swith would be the problem because everything else in the car was working. I had probed for 12 volts at the ignition module and it was fine.
The only problem was that the care was stalling.
Turns out that because there is a wire coming from the ignition switch thats intermittently not holding proper voltage, the PCM and BCM were not getting the 12 volts from the ignition switch and causing the car to stall.
It took 4 months of seeing the car sit, but I can say I'm a happy man having fixed it myself.
Those con boys at the Chevy dealership can kiss my ***!