Garbled X11 screen output

Started Ubuntu 11.04 from a live CD (downloaded 2011-04-28 over BitTorrent). Already upon startup, the screen output has some minor irregularities, but I can get to the point where I can click "Try Ubuntu". The desktop comes up, with similar irregularities in the output. When I then click the Applications menu, I get a black square where the menu is supposed to appear (some parts of the menu do appear briefly); as I move the mouse to the next item, another black square appears where the second menu should show up (the first black square remains).

I then hit ESC and click a random location near the middle of the screen; this results in another black square from the top-left corner to the pointer location.

On one instance I tried opening a window (Preferences), same here: parts of the window seem to appear briefly and are then obstructed by garbage.

Bottom line: on the graphics card used, Natty is unusable. I have successfully run Karmic, Lucid and Maverick (each from both the live CD and installed on disk) on that same computer, so it's definitely a new bug introduced with Natty. On my other computer (which has a different graphics card) Natty runs OK (only issue there is that the mouse pointer disappears when I click a menu item and reappears as soon as I move the mouse).

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: linux-image-2.6.38-8-generic 2.6.38-8.42
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic i686
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.23.
Architecture: i386
AudioDevicesInUse:
 Cannot stat file /proc/4551/fd/43: Stale NFS file handle
 Cannot stat file /proc/4551/fd/44: Stale NFS file handle
                      USER PID ACCESS COMMAND
 /dev/snd/controlC0: ubuntu 2880 F.... pulseaudio
CRDA: Error: [Errno 2] No such file or directory
Card0.Amixer.info:
 Card hw:0 'ICH5'/'Intel ICH5 with AD1980 at irq 17'
   Mixer name : 'Analog Devices AD1980'
   Components : 'AC97a:41445370'
   Controls : 46
   Simple ctrls : 32
Date: Sat Apr 30 10:01:11 2011
IwConfig:
 lo no wireless extensions.

 eth0 no wireless extensions.
LiveMediaBuild: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
MachineType: FUJITSU SIEMENS SCENIC W300/W600
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: file=/cdrom/preseed/hostname.seed boot=casper initrd=/casper/initrd.lz quiet splash -- maybe-ubiquity
RelatedPackageVersions:
 linux-restricted-modules-2.6.38-8-generic N/A
 linux-backports-modules-2.6.38-8-generic N/A
 linux-firmware 1.52
RfKill:

SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/03/2003
dmi.bios.vendor: FUJITSU SIEMENS // Phoenix Technologies Ltd.
dmi.bios.version: 4.06 Rev. 1.07.1567
dmi.board.name: D1567
dmi.board.vendor: FUJITSU SIEMENS
dmi.board.version: S26361-D1567
dmi.chassis.asset.tag: YBES094579
dmi.chassis.type: 6
dmi.chassis.vendor: FUJITSU SIEMENS
dmi.chassis.version: SCEW
dmi.modalias: dmi:bvnFUJITSUSIEMENS//PhoenixTechnologiesLtd.:bvr4.06Rev.1.07.1567:bd12/03/2003:svnFUJITSUSIEMENS:pnSCENICW300/W600:pvr:rvnFUJITSUSIEMENS:rnD1567:rvrS26361-D1567:cvnFUJITSUSIEMENS:ct6:cvrSCEW:
dmi.product.name: SCENIC W300/W600
dmi.sys.vendor: FUJITSU SIEMENS