Pontiac Repair: 2001 Pontiac Bonneville Charging Problem, powertrain control module, pontiac bonneville

Question
If I drive in town more than 1 time per week and do not take the car out of town, the next time I try to use the car it is completely dead.  When I'm driving the gauge indicates that all is charging fine.  I've replaced the battery and it still continues.  I've had the battery, cables, starter, alternator, etc tested 3 times and everything checks out ok.  The garage keeps asking me if I'm sure I've shut everything off, which I do, including the radio.  If I drive the car out of town only, with no in town trips, the car goes for months with no issues.  As soon as I do more than 1 day of in town driving, without an out of town trip in between, the car dies.  This last time I did 3 consecutive days of in town driving and then the car sat for 3 days.  On the 4th day I went to use the car, to go out of town ironically enough, and it was completely dead.  I'd think there is a charging/alternator issue but whenever I drive the car the gauge has an acceptable reading.  Any thoughts on this frustrating issue?  The car is in perfect shape otherwise and only has 48,000 miles so I'd like to keep it around.  Thanks!!

Answer
Hi
I have been doing alot of thinking and talking to friends about this.
Not having the car here it is REALLY hard for us to properly diagnose your concern.
That being understood, here are the 3 things we came up with not in any order
Check the DIM fuse in rear fuse box. This is the Circuit that the Aternator uses to sense.
Have a diagnostics run on the PCM (Powertrain Control Module) and see if there is an Alternator code stored. It will store alternator codes and NOT tun on the Service Engine soon light. The PCM recieves the Alternator field duty cycle signal and turns on (or off) the Generator lamp on the Instrument cluster from this signal recieved. If possible have the tech "watch" the data of the duty cycle and the Alternator on/off with a scan tool. You MAY catch something.
This one could be done at an Alternator or electrical speciality facilty, have them REMOVE the alternator and load test it on the bench. Make it work. At the same time, have them preform a load test on the battery with a "carbon Pile" to really check it's reserve.
Of course I always say to check the basics first, the power cable from the Alternator to the underhood fuse box and then rearward. Check the Alternator body, make sure it is grounded GOOD to the engine.
Most guys I spoke with seem to think you have an intermintant Alternator problem.
Also if time permits, you could leave it at a repair shop and they could run a parasitic draw test on the Electrical system. You MAY have a module that is staying awake occasionally. This will run down a battery overnight. A Parasitic draw could isolate that problem used in conjunction with a Tech II scan tool. Just suggestions. See Modules use a small amount of battery reserve power to keep internal clocks and settings. Like preset radio stations or remote keyless entry frequencies or the clock for radio. After a preset time, they "go to sleep" (similar to your screensaver on power on your PC). If one were to stay "awake" it would use MUCH more power and would run down a battery, just like if you left the dome lamp on all night.
Well there you have it. Our answer!
I really wish we could have the car here, but you understand.
Find a good Automotive Electrician in your area. Explain all this to them. It is better to spend this money now and continue to have a good perfect car for a good long while.
I Hope this helps.
This was just advice from experience.
Good Luck, and keep me posted.
Paul