I have a simple Ubuntu server on a Dell desktop serving only the basic services (web server,mail,ftp,ssh etc). Always up to date.
I've been experiencing two problems that I think could be linked:
First of all, sometimes the fan goes off speeding crazily with apparently no reason. If i check with htop, I see the CPU is used at 0% by all processes. Maybe some processes could be hidden?
Second, at very irregular intervals, the computer just turns off (doesn't shut down properly) and the front led start blinking orange. The strange thing is that the BIOS is set to restore the state of the system in case of power fault (ie. it turns automatically back on if the power goes out).
What could be causing this? I've been thinking for a long time of a security problem (thread here) but couldn't find anything so far, so I'm thinking it could be a hardware problem or something to do with the power supply (the electricity infrastructure in my house is very old and sucks)
I'd appreciate any suggestion or comment.
 Just now, the computer went off. I had htop open on a terminal, so I'm attaching its screenshot. That --purge --config=/etc/denyhosts.conf looks suspicious to me.
And the led on the lan cable attachment keeps going on and off as if it were still exchanging info. the system seems completely off though!