Dell Latitude E6400 intermittently no video and chaotic POST response

I have acquired an used Dell Latitude E6400 laptop that has a weird problem.

A couple of times, all of a sudden the video faded away, and didn't come back for dozen power cycles. Screen/inverter/video chip were ok though, as colors displayed just fine when I press D + power. Also, when video faded away, all LEDs on the board turned off, except those on the back of the screen.

Also, another time, it didn't POST at all. When powered up, it would spin drive and fan for a few seconds and shut down, leaving some LEDs on (HDD + WiFi) for another few seconds, after which the LEDs turns off, flickering for a split second. A couple of times, after all that, it even kept blinking all 3 keyboard LEDs = System board error. This happened when on battery, AC or both, so I don't think si power source. I think is either the power circuit or the logic circuit on system board.

Right now the laptop is fine. It passed all diagnostic tests, and power cycles are working perfect. But I'm sure the issue will pop up again at some point.

This is most likely a motherboard issue, but I'm wondering if it can be fixed without board replacement, if a simple fix is applicable.

So I have a couple of questions about general hardware design of system boards:

1. Is this common behavior for BIOS chip failure?

2. Vaguely, what is the control path between BIOS software and the system board LEDs? What would cause them to flicker while nothing else would? Could corrupt BIOS cause that?

3. Are there separated power "rails" for CPU-HDD/video/etc and could one work without the other?

As you can tell, I'm having a hack at it and I'm attempting to do a guesstimate exclusion diagnosis.

Thanks