Thinkpad 380XD bizarre power failure

[PhaseSeeker]

Hi, like it says in the title, I've got an old Pentium MMX Thinkpad 380XD laptop which has a puzzling power issue to say the least. Basically:
- On a "cold" boot (as in when the thing has been sitting a while, even if the ambient temperature is high) it powers up perfectly fine
- After 15/20 minutes of using it, it just drops completely dead. No charger/battery LED, no response when sliding the power button, nothing.
- If it sits a while, it starts working again.
This is 100% reproducible every time
Now, this to me sounds like a thermal/mechanical issue, as in some trace or component which gets disconnected with thermal cycling, however 1) the solder on this thing looks perfect, no cracks ecc that I've been able to find and 2) The DC-DC module is pretty inaccessible with the thing assembled, and there's no schematic, so it's gonna be a nightmare to debug.
I thought it might possibly be the CPU overheating causing a protective shutdown, but the thing is COMPLETELY dead, not even drawing any current to charge the battery, so it seems unlikely.
The electrolytics on the DC-DC converter are new but even the old ones tested in spec, plus I would think they would be a problem when cold, not the opposite.
So, basically, is there anything obvious I should check? Or should I just accept my fate and try to make and extender board to be able to probe the DC-DC converter module outside of the computer?

[LayaVulpes]

This is kind of bizarre and as you said, it will be very hard to diagnose. The only things, that you could check is if laptop behaves the same without battery (you can check if its the same without cmos battery as well) and if it still behaves the same you can try to unplug a hdd or boot to bios and just wait if it shuts down there.
That way you can check if its only under some load or if it shuts down no matter what. You can maaaybe check ram (because sometimes old computers can shut down without a warning if the ram is bad, but its rare and highly unlikely).

[PhaseSeeker]

The battery was the first thing I suspected, but there's no difference with it inserted or removed. I haven't tried without the CMOS battery though. No HDD doesn't make a difference, RAM iirc was tested with memtest86 with no issues but I'll check again.