[seems solved - still in testing]
First, thanks for the answers and the help, and sorry for the delay in my reappearance here.
Yes it is a 64 machine.
About the xorg. well, i can post it, but i think it is not going to be necessary right now.
About the crash, well, as i mentioned in the original message, the computer needs the hard reset to be turned off and on again.
About video on board, no, i don't. The motherboard is an Intel 975XBX.
About the temperatures, i use to monitor them with lm_sensors and gkrellm, and from time to time, confirm the readings with the BIOS readings, and they where accurate.
Well, after a few days of testing, the things seem to be more stable.
This is what i have done and tested.
In the same computer:
- ati-drivers (problem reported here)
- radeonhd (same problem, less frequent)
- vesa (no problem at all)
- memtest (all ok)
So, the drivers with more options, and more demanding causes more problems.
I haven't used cpuburn to test, but since my system use to run boinc the whole day, I think that is quite a test. This computer is 1 year old, more or less, and has been running boinc for almost the whole year.
From another computer when the system dies.
- Ping to this one and it was working. ( Test made when the problem happened again ).
- Connected through ssh and checked logs; no problem at all reported in any of them.
- Connected through ssh and checked about the X being loaded and working. They were not in memory. Neither fluxbox
So, from the other computer i did reboots or halts when necessary.
With those tests i confirmed that the system doesn't really dies, just the graphical system.
The next test i did, was to connect a fan close to the graphic card and in front of the southbridge and then the system seems to be more stable. Obviously this point out to be a temperature problem, but the thing is the system did not report any excessive heat, not in the core, not in the bridges. And the graphic card was never hot enough that i couldn't touch it.
Any way. The system seems to be more stable now. Only one problem starting the X system today morning, but again, no logs. This time i could not connect from another computer, so i had to reset manually. May be it was related with the original problem, may be not
Thanks again for the help and the answers, I will keep doing some more tests until I'm convinced that the system is really stable, but if you have any theory or idea, I'll be glad to hear it.