I installed yesterday Ubuntu 12.04 on my laptop, which was running on Debian 7 until then. I installed MATE (a GNOME 2 fork) on Ubuntu 12.04 because I can't stand GNOME 3 or Unity. My Debian 7 ran on GNOME 2.

And here's a problem that happened on both operating systems, though differently.
At any random moment, the mouse's left button stops clicking. And it probably isn't related to the mouse I use since the touchpad is always simultaneously struck.

It happened twice since yesterday and it doesn't go away until reboot. Actually, I was talking to a friend and as I couldn't left-click anymore, I managed to get around with the right button only (though much much slower). And then the right-click button died too, at which point I hit ctr-alt-F1 to enter the Command Line interface. I logged in and typed in
kill -9 -1
which usually does the effect of a soft reboot, resetting the Graphical User Interface and bringing me back (after pressing ctr-alt-F7) to the log in screen. But instead, the computer just froze there and I had to hold the power button to reboot.
And here I am a few minutes later.

Actually, under Debian, it happened often for the left-click but the problem cured itself after a minute or so.

Can someone pull me out of this? What is this plague? And please, please, don't try to convince me back into Unity.

Thanks a lot!