X.org stalls - problems with NVIDIA graphics driver?

[Thomas Sperre]

When I installed Dapper, I wanted to enable the graphics acceleration that 
should be possible with my graphics card so I installed NVIDIA driver. I like 
eyecandy, and I have enabled shadows etc using «Composite» in xorg.conf. 
Also, from the list of screensavers, I selected some that used some fancy 
OpenGL effects. All these 3D enabled effects seem to work just nice in the 
beginning; but fail after some time, usually ~30-40 minutes but can be much 
more and sometimes less.

 I use partial transparency for the windows that don't have focus, and shadows 
for all the menus and windowframes. As with the screensaver, this seems to 
work just fine until at some point the rendering goes extremely slow. It is 
faster than with the screensaver, but still painfully slow. We are talking 
maybe a frame a second. Sometimes when I select a menu or open a window only 
the shadow behind is rendered and then have to wait wait wait wait for 
content and then frame to appear. Even closing all active windows before 
logging out thereby restarting X is sometimes so slow I am starting to use 
the less gentle Ctrl-Alt-Backspace keycombination to restart X. 
  
My system information
I have a Point of View NVIDIA 6800GT 256 Mb graphics card, running Kubuntu 
Dapper Drake 6.06 LTS - Linux 2.6.15-26-386 running KDE 3.5.2, CPU: 
Intel(R)Pentium(R)4CPU2.80GHz at 2800 MHz (5605 bogomips), , RAM: 
2213/2535MB,

My xorg.conf file
Here are some parts of xorg.conf which may be relevant:
Section "Module"
  Load "bitmap"
  Load "ddc"
  Load "extmod"
  Load "freetype"
  Load "int10"
  Load "type1"
  Load "vbe"
  load "glx"
  load "v4l"
EndSection

Section "InputDevice"
  Identifier "Generic Keyboard"
  Driver "kbd"
  option "CoreKeyboard"
  option "XkbRules" "xorg"
  option "XkbModel" "pc105"
  option "XkbLayout" "no"
EndSection


Section "Device"
  identifier "NVIDIA Corporation NV40 [GeForce 6800 GT]"
  boardname "NVIDIA GeForce 6800 (generic)"
  busid "PCI:1:0:0"
  driver "nvidia"
  screen 0
  vendorname "NVIDIA"
EndSection


Section "Extensions"
  option "Composite" "Enable"
EndSection

Question here: it says busid "PCI:1:0:0" but I have an AGP version. Is that a 
problem?

I have spent hours reading through miscellaneous posts on nvidia driver topics 
mostly in various  fora. I cannot find a precise description that exactly 
matches my problem. I have tried some solutions from «related» problems but 
it appears not to have any effect.  I am not very skilled at this, and I am 
out of ideas to try at the moment. Is there anyone that has some suggestions 
how to fix this?

PS: yes, linux-restricted-modules is installed, according to adept

-- 
Thomas Sperre
thcsp at online.no
(+47)93448977