From time to time I can see error messages (2 times today) by dmesg

this seems harmless, but what does it mean ?

Hi...

I don't know if it relates directly but it's possible that this has to do with video acceleration, as mentioned here, here and here, in that order.

What is your Linux distribution (and version) and video chip in particular?

If you're able to use Nvidia's proprietary driver, I would recommend that.

Regards...

Get the firmware and copy it in /lib/firmware/.

my graphic card is a GTS 250
I have extracted the firmware files (using one of the links of ardvark71). I have copied them in /usr/lib/firmware/nouveau, as indicated in the nouveau website.
The messages in my previous post appear sometimes when I use vlc to see some videos. I tried this with the firmware installed. This freezed completely X, keyboard an mouse did not work anymore. I could only get back to the console via ssh from another computer, by killing many programs. I think that I will go on without the firmware, since I was aware of this problem only by inspecting dmesg output.

What is your video output in VLC set to?

I have "Overlay" and "automatic"

It probably switched to vdpau when firmware was loaded. You could try other outputs and leave firmware loaded, not sure, but I remember reading somewhere it helps with power management.