I am having a frustrating issue here. My computer is crashing. It happens when I'm browsing any site on web using Firefox and there are other windows and applications running, such as torrent client, LibreOffice writter, file manager, Dropbox.

It happens when I open a new web site or a new tab on Firefox. The computer gets very slow, the window goes grayed out and then I get a black screen with some few sentences. But this black screen disappears very fast and all I could read in this black screen so far is something like "disconnected from Plymouth". The computer reboots and I have to start a new section. I've lost unsaved data many times.

I have no idea why it happens or what could possibly be causing it.

What could it be?

Is there a way to discover what drove my computer to this fatal crash? Is there any diagnosis tool or crash log tool that I could use?

Is there any setting I could use to avoid this frustrating and continuous system crashing?

My system's details:

OS: Ubuntu 12.04 LTS
Processor: Intel® Core™2 Duo CPU E4500 @ 2.20GHz × 2
Graphics: Intel® 945G x86/MMX/SSE2
OS type: 32-bit
Disk: 236.4 GB