MBP random shutdown even with power cord plugged in

Question

My MacBook Pro (original) has now randomly shut down 5 times since I installed 10.5.6 the week before Christmas. 4 of those 5 times happened with the power cord plugged in, the green indicator light on and the installed battery fully charged. I have two batteries and it has happened with each of them at least twice. Yesterday I got my first shutdown while on battery only. After each shutdown I was able to restart the computer and it operated normally, even the one on battery only.
These shutdowns are fast and complete. There is no warning of any kind. The screen just goes blank. It is not sleeping, there is no pulsing light on the front. Using the start key begins the process from scratch with the Apple "startup chime."
The occurrences have gotten more frequent. The first two were within 24 hours of when I installed 10.5.6 and they happened within several hours of each other. Both of these were with the power cord attached and fully charged batteries. I swapped the battery after the first one. When it happened again, I swapped them again.
The next time I had the problem was 1/30 (last Friday). The only significant software installation I had done between installing 10.5.6 and then was to install iLife '09 on 1/28. All software updates had been installed in a timely manner. Friday the laptop was plugged into external power and the battery was fully charged, but I don't know which battery was installed.
2/1 I had an issue where my computer couldn't locate the OS file on startup so I did a archive and install. It operated normally since then until yesterday, 2/4 when I had two shutdowns. The first I had the computer on external power and a fully charged battery. The second was the ONLY time I have had the shutdown occur while on battery power only. I was on a flight when the second shutdown occurred. I had used all the power in the battery that was installed when the first shutdown happened (down to about 11 minutes remaining). I then swapped batteries without shutting the computer down. The system rebuilt itself normally and I began working on iPhoto. About an hour later it just shutdown. I was able to restart the computer normally and used it for another 45 minutes or so before I shut it down for landing.
I called Apple Care this morning and they advised me to get an appointment with a Genius. I resist being without my laptop but will try this and let them look over the console log to see if they can find anything.
I have done all the basics, reset the PMU, zapped the PRAM and repaired disk permissions. The next thing I am going to do is to calibrate both batteries. I doubt this will have any impact on the issue because most of the shutdowns have occurred with external power connected. I just haven't done this in a while.
Any ideas out there? Anyone have a similar problem?
I have another issue on my Mac Pro since installing 10.5.6 and my wife's iMac G5 has a completely different issue since installing 10.5 (I didn't install this OS until last week so I could get the new iLife on it). Seems like this latest OS version is a bit buggy or at least incompatible with a lot of Apple's products in different ways.
Fight On!

Answer 1

UPDATE:
Service was due and now that I saw the machine fail 4 times in a day doing the same process I knew I could duplicate the issue on demand. That's a big bonus.
Called Apple Care yesterday morning and the guy I spoke with read over the notes on my case after I told him what happened the day before (black friday, 4 shutdowns). While he didn't transfer me to a Product Specialist, he did talk to one for quite a while (I was on hold for about 20 minutes while he did this). He said I could ship it to Apple and they would look at it or I could take it to an Apple Store or Authorized Service Center. Thinking this was the next step I had secured a time at the Carlsbad Apple Store for late that afternoon (my usual & closest store, Escondido, was booked all weekend). They told me to take it in and reference the case number. The Product Specialist thought it sounded like a logic board and would duly note that diagnosis in the case history.
After the conversation ended I decided to make sure I could cause the failure. So I started the process (threw out the face.db from the iPhoto Library package and opened iPhoto whereupon iPhoto begins a scan of the images for faces. Having over 12,000 images gives it a lot of work). This time I decided to monitor the workings of the laptop using iStat Pro (freeware widget download & I recommend making a small donation it's so good). I kept track of the processes at 5 minute intervals, the CPU usage, numerous temps AND the fan speed. The latter seemed to be the critical. Shortly after the fan speed got to 4700 the shutdown occurred. It took 36 minutes from the time I started. The fans kind of confirm to me that it could be the logic board.
I took the laptop to the Apple Store and started the process in front of the "Genius." He read through the case as well as the printed copy of this thread and other notes I brought from previous shutdowns including the data I mined from iStat Pro. He was unimpressed and thought it was a non-issue. I guess that's only fair because I was unimpressed with him. He figures it's my system because of a software conflict somewhere. His answer was to do a clean install and reinstall all the software from the original disks. I told him I have spent over 50 hours on this issue to date and that what he was suggesting would take at least that much and guarantee nothing if it's a database issue...duh! Disagreement. Oh, and he also doubted it would fail just from making it work.
43 minutes after starting the process, well after my "Genius" went home for the day (I was his last appointment), the failure occurred. The "Genius" he handed me off to was working with someone else next to where I was sitting. When the fans hit 4700 rpm I told her it was going to happen soon and as I finished my sentence the failure occurred! Cool.
They have the machine and I'll let you know what they do.
Fight On!