X61t turns itself off randomly in winXP

[cogit0]

I have a X61 tablet running Windows XP Tablet edition. The problem started when I had my system on battery power, and then randomly rebooted itself. After repeating that twice, it was dead. I was able to turn it back on after removing the battery and AC power and letting it sit for a few hours, at which point it turned back on, stayed on for 5 minutes, and repeated the whole thing. After the initial failure, it failed multiple times, both under battery, AC with battery in, and AC alone. Each time, it rebooted after being in windows for a brief period of time (varying from in process of boot to 5 minutes) and shut itself off. While under battery power, it attempts to restart itself, and shuts off during POST.

Since then (and $500 later), I have replaced the system board (and CPU), the RAM, the hard drive, the battery (8 cell), and the AC adapter. It still has the same screen module, wireless NIC, modem, and a few of the other odd components. I think importantly, it still has the same DC power connector. Unlike most other laptops, the X6x has a 6 line cable that goes from the DC power in and feeds into the system board. I have ordered a replacement one and should be getting a new one of those soon.

After replacing all of these parts one at a time and testing in between, the system looked like it was working again. It did fine for about a week, until while under battery power it rebooted itself again. Again, the same symptoms have been occurring: rebooting while under battery or AC power, getting caught in a POST reboot loop, not able to power up while just under AC power. For a week or so I was able to keep it working for hours while just under AC power (with the battery in), but rapidly deteriorated after I closed it and had it in sleep mode off of battery power. I have replaced pretty much everything that I can think of that could cause this kind of problem, and it is still happening.

What kind of hardware issue would cause these symptoms? We originally thought that it was a power source problem, that a faulty AC adapter caused a short in the system board and then faulty battery controller as well, but I've replaced all three of them and am still having problems. Would a grounding short do this? I would think that if it is a grounding short, it just wouldn't be able to boot at all; instead, it works for a little while, and then dies.

Any thoughts as to what could be causing this? I've stumped all of my other IT buddies. No one has heard of anything like this before, so hopefully someone here may be able to point me in a better direction.