Originally Posted by Rian D. Spivak
On a Thinkpad R40, the mainboard pretty much comprises most of the components of the system. As far as the traditional responsibilities of the power supply, this is split up between the mainboard and the power adapter. The power adapter's job is to convert the power from AC to DC at a specific voltage. Once the power is fed into the machine, it gets converted to right voltages the different components of the computer needs.
The small explanation is yes, a mainboard replacement replaces the components that handle the job of the power supply.
I'm not sure exactly what might be causing your current problem and as you stated is hard to duplicate. Chances are though, if it was a power adapter problem, then the battery should kick in to compensate for the lack of power. Does the computer turn immediately after it shuts down?
Sorry, your very last sentence didn't make much sense to me... but I'll describe it best I can.
In a nutshell, now and then power will completely cut out. It does not suddenly switch to battery power then die, the computer will outright go black. It doesn't go through shutdown, nothing. The "power" indicator next to the battery light is dead, and half the time at this point it will stop even reading that the AC adapter is plugged in.
At this point, plugging in or unplugging the AC adapter will (sometimes) not even display on the battery light on the R40. Removal and reinsertion of my battery has sometimes fixed the job, but in all honesty, it seems like a coinflip as to whether or not it'll actually start recognizing that something, ANYTHING is powering the system, be it a brand new battery or power adapter.
Occasionally after these shutdowns, the power button will not respond for several moments... and if it does, it will be immediately followed seconds after by another shutdown.
EDIT: Alright. Nearly twenty shutdowns today. Only my persistence and the DVD menu for Borat kept me going. Bringing it in tomorrow. Here's hoping that pink elephant syndrome doesn't occur again (they already seem to think I'm crazy down there). Here's hoping we can actually nail the problem this time, instead of just replacing the mainboard a third time and crossing our fingers.