Macbook 2008 4,1 serious problems after 10.6.8 and 1.4 firmware update

Question

I'm having a problem with my 2008 2.4Ghz Macbook. After doing a fresh install of 10.6.0 with a Retail DVD, the MB was working perfectly, absolutely no problems at all. I then ran Software Update and installed all the recommended updates to 10.6.8, and I only noticed in passing that there was a Firmware update to do too, as reported by Software Update. I ran that as well and, IIRC, it was applied after fully updating to 10.6.8 and after rebooting twice. The MB rebooted, ran the Firmware update (which seemed to complete fine, as the progress bar completed normally against the grey screen), then rebooted. Again, IIRC, after running the Firmware update, and I'm now at 10.6.8, I started a Repair Permissions and left the MB for a while. It was the next day, maybe 15 hours later, when I went back to the MB and it was in Sleep. I pressed a key to wake it and this is when the weirdness started.
The screen was "fuzzy", sort of out of focus, though I could see the desktop, and what I could see was: the Disk Utility window open on Repair Permissions (which had completed normally) and several Vertical Segmented Bars running across the bottom part of the screen, at about the height where the Firmware Update progress bar was. These bars were filling in from left to right. IIRC, it may have looked a little like when you wake from Deep Sleep (or Hibernate, or whatever it was called). Like when you Wake from Sleep after the battery has run out and then you plug the AC adapter in.
Anyway, before the Vertical Segmented Bars progress was completed, the screen went black. Pressing keys didn't bring the desktop back.
I did a forced shutdown, reset the SMC, then rebooted. No startup chime. The Superdrive, hard drive and fans spun up, but the screen remained black. After a few reboots, I got a desktop back, and tried to reinstall from both an external HD (with Restored disk image) and the Retail Snow Leo DVD. Both times, the MB booted to the startup screen: with the external HD, I held down Option and chose the Snow Leo install; with the DVD, the MB started normally while I held down the C key. The boot started from the HD and the DVD, but after about 60 seconds, with Apple logo and the spinning "cog wheel", the screen went black. And seemed to shut down. With the external FW hard drive, the drive made a loud click, its LED went off, then it came back on after a few seconds, as if it had lost all power to it, then the LED lit up when power returned to it from the shut down MB. Anyway, the installs failed.
I then attached the 2.4 MB to another MB (2.1) in Target Disk Mode and it booted fine into TDM. It showed up on the 2.1's desktop, I ran Check Disk and it checked out okay. I then looked under Startup Disk and the 2.4's disk was listed there, with 10.6.8.
I then booted the 2.1 from the 2.4's disk and it booted fine. I ran check Permissions, and a huge list of permissions to repair came out. At this point, I've left things as they are. The 2.1 is booted from the 2.4, the latter used as external boot HD.
I then replaced hard drive and RAM. Tried to boot from DVD and install, got to Apple logo and spinning cog wheel, but after about 2 minutes the screen went black.
I then connected an external monitor and rebooted from the DVD. The external display received a video signal and I got to the Snow Leopard install screen. All this time the Macbook display has remained BLACK.
I went to Disk Utility and reformatted the new drive, then started the installation, which completed fine. The Macbook runs normally connected to the external display.
Any thoughts?
Why would the Macbook display work fine at boot (Apple logo, etc.), but then cut out after about 2-3 minutes?
Why would the Macbook screen remain black while I got a video signal on an external monitor?
Does this sound heat-related in some way? I should note that while booting from the DVD and now installing Snow Leopard, the fans are going pretty fast.
I just want to point out that this was an absolutely pristine, impeccable Macbook until these recent problems. It is in fantastic condition, none of the usual chips out of the top case and trackpad area. After the latest fresh install of 10.6.0 it was perfect upon reboot. Fast, perfect Airport and Bluetooth, FW and USB worked perfectly with a FW drive and USB mouse I hooked up. The update to 10.6.8 via Software Update seemed to complete fine.
Then all the problems started AFTER THE FIRMWARE UPDATE.
I don't know how much stock can be put in the firmware update being the cause of the problems, but the sequence here would bear that out. These are also somewhat unusual problems, at least in my experience.
One of the odd things was when I first went to wake the Macbook after the firmware update: the "fuzzy", out-of-focus screen and the segmented progress bars. And it was just before the segmented bars completed progress from left to right that the display cut out and went black. And from that time on, I've had the aforementioned problems.
Any ideas or links to similar problems would be greatly appreciated, e.g. possibly firmware related, or where the Macbook screen works but goes black after a couple minutes. And now that I've been able to connect to an external monitor, it would seem that the video signal is okay too.
Thanks.

Answer 1

Just to update.
After leaving the Macbook unplugged and without battery for a few days. I reset the SMC and, with no battery installed, I've just now rebooted and:
1) the first time, it booted, but no chime, and when the display turned on, once again I got the "out of focus" desktop AND the segmented vertical bars on the desktop (and the bars were filling in from right to left); the display went black after about 10-15 seconds;
2) I shut down and swapped some different RAM, then reset the SMC again and booted without battery. This time I got the boot chime and the Macbook booted to the Apple logo screen and spinning cogwheel. After about 2 minutes, the cogwheel stopped spinning, as if to boot to the desktop, but then the display went black. The front light started to pulse after about 10 seconds, and the Macbook appeared to be in Sleep.
3) I woke the Macbook from sleep by hitting a few keys and this time it went to the desktop. This is the first time it's gone to the desktop since the problem started.
I had enough time to see that everything appeared normally on the desktop, such as the time and date, Airport was connected, etc. And then the display went black again, after about 10 seconds.
At this point, I can now always get to the desktop, but the display goes black after about 10 seconds. I have just enough time to press a few keys and the Macbook does respond.
4) I shut down and reinserted the battery. Here's where new problems show up.
With the battery inserted, and the Macbook in sleep, if I hit a key, the DVD spins up as the Macbook is waking, BUT now the front sleep indicator light FLASHES (blinks) 5 TIMES. No beeps.
The Macbook still gets to the desktop, like before, and the screen goes black after 10 seconds.
I found some info on the Apple site re. 5 flashes (Article 21502):
If you don’t hear beeps, but the sleep indicator light flashes five times when you start it up or wake it from sleep, its battery may need service. Take the battery to an Apple Authorized Service Provider, such as an Apple Retail Store.
For more information, see the Apple Support article below.
You can continue using your computer before you service the battery. Use a different battery, or connect the computer’s power adapter and plug it into a power source.
Note:If you use your computer without a battery or with a faulty battery, you could lose all unsaved changes if it’s unplugged while it’s on.
BUT, the battery is not defective, as it works okay in another Macbook.
I should also point out that the problem with the 5 indicator light flashes or with the battery did not exist before the 10.6.8 and the Firmware 1.4 updates.
Any ideas?