I have a Lenovo R61 circa 2008 that has had this abrupt shutdown problem for the last year. "Abrupt" as in there is no OS shutdown, no warning, just POOF! it is off. As if abruptly unplugged from all power sources.
I've replaced the CPU/GPU fan+heatsink twice already. The last replacement fixed the problem, but only for a few hours.
The batter pack is dead as a doornail. However, I was running with no battery and it was fine for several hours.
When I unplug from AC, and plug back in, it is fine.
I tried holding the power button down, while unplugged from AC, for about a minute. That seemed to fix it, but once again, after a few hours, it started doing the abrupt shutoff again.
Now, several hours after that, I cannot get it to stay on for more than about a minute, no matter what I do.
Is there any fix for this? How much is a mobo replacement? Would that fix it? Any other info or diagnostics or quick fixes I can try?