Computer always powers itself up after shut down

[mamboknave]

Board: ASrock H87M pro4, with the latest BIOS P2.0; Ubuntu is 12.04 64b w/ Kernel 3.5.0-45

1) I do shut the computer down from the menu OR with the command line "poweroff" OR "shutdown -P now"

2) It totally shuts down, all fans, all leds, the screen.

3) It stays off for a few secs, fooling me.

4) Then it restarts, powering itself back on.

The only way to impede the restart is to switch off the power supply on the back of the case at 3). Doing so, it still tries to do 4). You hear the fans starting, but the little charge remaining in the the power unit gets discharged in a moment and it stops there. It's off.

This problem started around mid Jan (after months of regular functioning), when I updated the Linux Kernel to 3.5.0-44.

It's certainly caused by the Kernel as it does NOT occur with v 3.5.0-41 and below. It occurs with Kernel 3.5.0-44 and above.
I Cannot say if it occurs with 42, 43.

It was verified this problem does NOT occur with Windows OS.

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. It seems that your bug report is not filed about a specific source package though, rather it is just filed against Ubuntu in general. It is important that bug reports be filed about source packages so that people interested in the package can find the bugs about it. You can find some hints about determining what package your bug might be about at https://wiki.ubuntu.com/Bugs/FindRightPackage. You might also ask for help in the #ubuntu-bugs irc channel on Freenode.

To change the source package that this bug is filed about visit https://bugs.launchpad.net/ubuntu/+bug/1275590/+editstatus and add the package name in the text box next to the word Package.

[This is an automated message. I apologize if it reached you inappropriately; please just reply to this message indicating so.]

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[penalvch (penalvch)]

mamboknave, thank you for reporting this bug to Ubuntu. The Quantal enablement kernel reached EOL in August 2014. For more on this, please see https://wiki.ubuntu.com/Kernel/LTSEnablementStack . See this document for currently supported Ubuntu releases: https://wiki.ubuntu.com/Releases

Is this an issue in a supported kernel/release? If so, could you please execute the following command, as it will automatically gather debugging information, in a terminal:
apport-collect 1275590

[Launchpad Janitor (janitor)]

[Expired for linux (Ubuntu) because there has been no activity for 60 days.]