A21m strange power off

[tincus]

Hi people,
I've owned a 2nd hand a21m PIII 700 and works fine.
The problem:
The machine shuts off randomly. BUT only makes it under Windows, while surfing some heavy duty web sites ej http://www.speedy.com.ar or http://www.youtube.com, in Firefox, IE, Opera, any browser.
The first thing I check is temperature, the cpu is on 48° aprox with arctic silver, that I installed.
With battery, with AC power, the same thing. Windows XP, windows 2000, tried too.
The battery is almost died (20 minutes autonomy), but with AC power this may work well? or not?
Any help?
Thanks so much, sorry for my english

[ryengineer]

Youtube website gives me a hint that your machine perhaps is overheating. Monitor it's temperature to be sure if this is the reason.

[tincus]

In youtube cpu temp goes to 50º aprox, it's normal for a mobile Pentium III?

[tincus]

I switched Speedstep to auto, and the CPU mode to Auto (medium) instead of Fixed max, and seems to work fine

[tincus]

Failed it again . I saw that the CPU fan only starts in the IBM thinkpad bios screen, afther that, the fan disables and never start again... that's normal?? thanks.