I experienced this many times, especially watching flash content on Youtube and Vimeo, but not always. At times, I get black screen without a reason (at least with flash player being not shown).

I took a photos. This one appeared when I was watching Youtube/Vimeo:
http://wstaw.org/m/2011/06/10/20110609_002.jpg
(it’s a kernel panic and I had to reset my PC)

This one appeared without flash player but I could return to the system:
http://wstaw.org/m/2011/06/10/20110609_005.jpg

And this one appeared today when I was browsing the web:
http://wstaw.org/m/2011/06/10/20110610_002.jpg

It’s very annoying. Do you know what’s the reason and how to repair it?

At least one of those indicates problems servicing a paging request. Please check the memory usage of your system, and verify that your computer's memory is fully functional (memtest86 or similar should help here).

I would also recommend checking the SMART information on your hard disk, as one of those kernel panics involved a pipe used by Plasma, and the other panic involved functions used by ext4 (the file system).

I would also recommend checking the SMART information on your hard disk, as one of those kernel panics involved a pipe used by Plasma, and the other panic involved functions used by ext4 (the file system).

What is the SMART information?

One important note: the problems started to appear just right after fresh installation (with re-partitioning) of Kubuntu 11.04.
As for ext4, I used it for my /home. Previously it was ext3. But I did not convert the data stored in /home, but format the /home.

I did not have such problems with my previous systems.

OK.

Memory: ‘Pass complete, no errors’
SMART: ‘Overall health self-assessment test: PASSED’

Ok, what graphics card (and driver) do you have? Try disabling Desktop Effects to see if it has any effect.

Rather oldish Radeon Xpress 200 with open driver, radeon. I’ve disabled DE as you said and if the problem continues, I’ll let you know.

Hello again.

Two weeks went by since I turned desktop effects off. Since that time I have seen the black screen once. Do you think that graphic card drivers are broken? What do you suggest?

Maybe this matter is not related to the topic but today, watching DVD movie with VLC, Kubuntu has been frozen and I couldn’t do anything but restart. It happened twice so I gave up watching it in Kubuntu. The disc isn’t broken because I could play in under Windows.

cheers!

I would suggest you have a go at www.kubuntuforums.net and get help there - all friendly folk who run your OS