Freezes such as you have described can be both software and hardware related and as you have found sometimes frustratingly difficult to diagnose.
If this is a desktop PC look at your hardware-cards. For both laptops and desktops possibly acpi type issues.
It might be useful to temporarily simplify your configuration to have just the graphics card connected with a standard keyboard and mouse. All other cards should be removed.
For acpi related issues, try booting with
noapic nomodeset in your grub boot option. Its also worth trying
acpi=off although this could have other undesirable effects such as constant fan usage.
Also worth checking the bios version level and seeing if the vendor has a newer bios version. The readme notes should hopefully reveal if any newer version fixed crashes and freezes.
I note you have tried the standard 270 drivers but have failed due to freezes. Can you clarify if you had similar issues with the open-source driver? Obviously you will not get Unity during testing this.
Graphics freezing can be one of/or a combination of the driver/compiz/X/kernel
If you are willing to try any of the suggestions below first backup your system with a good backup tool such as CloneZilla. You will need an external media device to receive the image such as a large USB stick/drive or separate internal hard-drive.
Installing newer nVidia driver
Deactivate (uninstall) your current 173-nvidia driver using the Additional Drivers window.
There are a small number of important fixes primarily in the 275 stable but a small number also in the 280beta that fixed freezes - it is worth a shot to see if these apply to your graphics card. Unfortunately nvidia dont go into detail on which cards they specifically fix (readme.txt)
However - I would strongly recommend a backup unless you feel confident on reversing a nvidia install - especially since you had serious issues with the slightly older 270 drivers. I've used clonezilla countless times and it has always got me out of trouble. You do need a large external drive though - USB stick/external drive or a separate drive.
The latest graphics drivers have been packaged in the x updates ppa.
Note - this will lead you away from the standard baseline - if upgrading in the future
ppa-purge the PPA itself before upgrading.
You can also manually install the drivers from nVidia:
Try installing the latest nvidia stable 275 or 280 drivers -
32bit 280 drivers: ftp site and 64bit: 280 drivers: ftp site
CTRL + ALT + F1 to switch to TTY1 and login
sudo service gdm stop
To stop the X server
To run as root
To install the 32bit driver (equiv for 64bit) then reboot.
sudo sh NVIDIA* --uninstall
If you run classic Ubuntu with effects do you get the same freeze issues as standard Ubuntu? If you cannot reproduce the freeze with classic Ubuntu (no effects) then this will point you towards a compiz issue. I would raise a launchpad bug report with the compiz team.
If space is available (e.g. 20Gb), you could dual boot/install alongside the latest oneiric alpha. Obviously this will itself be unstable, but it will come with the latest X and Kernel. You may need to also install manually the beta 280 graphics drivers above since it probably will not be offered in the Additional Drivers window.
If during testing you dont see the same freeze activity you could try uplifting your X version with the x-edgers ppa and using kernel kernel 3.0 in Natty. Going this route is not really desirable - and could cause you upgrade issues in the future - and may have other unforeseen stability issue. Again, use
ppa-purge to remove the PPA.
Kernel 3.0 is packaged with the PPA - you'll need to install the headers as well as the kernel itself from synaptic BEFORE rebooting if you intend to install the nvidia drive later.
This is a testing ppa - do have a ready backup if you want to try this route.