I hade the sam problem and after much serching I have come across following solution i dont now if this will solve your problem, but apparently it have solved some peoples problem.
1) Edit in /boot/grub/menu.lst
sudo vim /boot/grub/menu.lst
title Ubuntu, kernel 2.6.15-27-686
root (hd0,6)
kernel /vmlinuz-2.6.15-27-686 root=/dev/hda6 ro quiet splash acpi=force
initrd /initrd.img-2.6.15-27-686
savedefault
boot
add acpi=force as above.
2)Go to the bios and se if you find something like Advanced->CPU Configuration->Execute Disable Function = Enabled
3)Follow the instruction in the link to update fglrx to the newest version
http://wiki.cchtml.com/index.php/Ubuntu_Dapper_Installation_Guide4)Edit in /etc/kd3/kdm/kdmrc
decomment following line
#TerminateServer=true
you will find it almost at the bottom of the file.
nothing of this work for me but maby it dose for you
But what did work for was following
5) Edite in /etc/X11/xorg.conf
decomment the Load "extmod"
Section "Module"
Load "i2c"
Load "bitmap"
Load "ddc"
Load "dri"
# Load "extmod"
Load "freetype"
Load "glx"
Load "int10"
Load "type1"
Load "vbe"
EndSection
good luck