Could be many things, we should start with this...
"Try Disk Utility
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair. Disk Utility checks and repairs the disk."
Then try a Safe Boot, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, reboot when it completes.
(Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive.)
One way to test is to Safe Boot from the HD, (holding Shift key down at bootup), run Disk Utility in Applications>Utilities, then highlight your drive, click on Repair Permissions, Test for problem in Safe Mode...
PS. Safe boot may stay on the gray radian for a long time, let it go, it's trying to repair the Hard Drive
Reboot, test again.
If it only does it in Regular Boot, then it could be some hardware problem like Video card, (Quartz is turned off in Safe Mode), or Airport, or 3rd party add-on, Check System Preferences>Accounts>Login Items window to see if it or something relevant is listed.
Check the System Preferences>Other Row, for 3rd party Pref Panes.
Thank you and sorry for this late reply, I was away and could not try what you suggested...
I have been trying that today twice, but after the safe boot (I do not have the tiger disks with me, so safe boot was the only option) the gray radian is on long enough for the computer to switch off by itself (12 mins the 1st time, 5 mins on the 2nd attempt... I have been noticing that activity time before auto switch off gets gradually shorter if I do not let enough pause time in between - temperature problem? but the machine seems cool, and I am not running any heavy soft).
Therefore I could not complete the suggested procedure... I will try again later and I will post eventual results.
Thanks again, g
Actually the problem started a few months ago when I moved the computer because I changed apartment. I do not know if having the computer lie flat for a while during transpo in the car may have displaced accumulated dust to a site where it causes problems.
I can say that for a month I just accepted the shutdowns, which were arriving pretty regularly after a certain time using the machine (20 mins max, and gradually less if I tried to turn it on again, but I could NEVER turn it on immediately after the shutdown - it would give the double spark turn on-shut off).
Then I decided to clean the inside via air can and vacuum cleaner and I did not experience the problem again for weeks (the machine has been staying on for 24 hours without probs at times).
Then all of a sudden it started again...
How can I check to be sure it is the thermal paste? Can I replace it myself?
Well, easiest way is to infer, get these & see if the CPU temps are inordinately high compared to the rest of the temps...
Get Temperature Monitor to see if it's heat related...
And/or iStat Pro...
If you have any temps in the 70°C/160°F range, that's likely it.
Sorry again for popping in randomly, but I am travelling too much in these days... in any case now I really doubt the problem is temperature-related.
The mac cannot be switched on anymore. As soon as I connect it to the power (directly to the main or switchin on the multisocket) the blue led goes on on for a second, I hear the sparkle-sound carachteristic of turning on the computer and that is it... the computer stays irresponsive if a try to push the start button.
I wonder what can be the problem, it looks like programmed obsolescence to me...