To achieve this do in System Settings > Boot Manager > Edit > add to Boot Options And restart to become active.
If not successful, remove the added option again.
(equivalent to this post)

i fully agree with you there maurice, and especially not from those who put so much effort in to helping someone!
sometimes the best help is not always the actual answer, but the way one gets there:
getting to know others along the way and learning new things. so, lisa: _O_

[I'm not keen on changing the DE Anglo, I'm used to KDE.]

A live distro to usb would be as temporary as possible.

[No problems with the machine running cool - it's a monster in size too. And no I can't borrow another card - all my friends are computer illiterates unfortunately. ]

The salient factor is the temperature of the card { Addendum : or if the card is still seated properly}

[ Being retired I use the machine pretty well every day - morning, afternoon and evening. Plus I'm one of those users who believe in switching it off when not in use so it gets switched off three or four times a day..]

I was referring to type of usage, e.g transcoding video's ,ripping cd's ,  massive number crunching, playing online games , or simple email, that sort of thing.....

This random behavior is strange.
What is approximately the rate of success/failure of this behavior?
And are there additional circumstances to get a successful shutdown?
Maybe it's wise to report this to the developers through Bugzilla.
I tend to think to a not set (=random) variable when the kernel is started.

Some thoughts...........
Try using a different DE (Desktop Environment ) like LXDE , a live one preferably,they run lighter and have different operational demands.
Your video card was introduced in 2007, : so is it running cool ( fan working?) , or is it overclocked?
Can you borrow a newer video card ?
For that matter , is your computer running cool?

Edit : can you give us a general idea of your usage of the beast ?

Hi Maurice,
and yes, owiknowi, its my job to apologize as I'm really not helping here as all of my ideas go in all kinds of directions, which is hard way to debug. 

I usually go to newegg to read reviews on a board or cpu as you get a rough idea of some problems that others are having, too.  There's a few people that have reported Linux and Intel board problems with ACPI.  I don't like to ask anyone to do a BIOS update, as sometimes it leaves them with a worse off working system, but there are some BIOS updates for your board that fix memory range overlaps.   But whether this fixes your shutdown problem is a big ?  It might get rid of the Soprano memory range error. 

Here is the newegg review page 3
One reviewer said they had to add, and I quote:
"Need pci=nommconf line on kernel boot line".  You might try this if you have not already. 

I was doing search with keywords: "dg33 + shutdown" and came up with plenty of hits. 

And some more of my tests to try that may or may not help is to disable graphics card in BIOS  and boot from the onboard graphics chip.  Good idea from others to try from live media Pardus boot. 
Another idea is when you do get a successful shutdown, be reade and hit the Esc key and this will bring you to terminal console  and it should say stuff like going to runlevel 6, disabling services, unmounting file systems, something about dbus,  and making file system "read/write" or maybe it says "read".   You might catch an error there and you can let us know. 

No , have a look at http://www.pardus.org.tr/en/pardus/indir/
There is a "2011 Live" section there

I would also experiment with services.  First, I would disable acpid and shutdown computer.  It will come back on upon restart:
Another test would be to enable service hal:
and try shutdown.  
It won't start again upon restart, but will if you do want to try it upon restart and shutdown then:

No , have a look at http://www.pardus.org.tr/en/pardus/indir/
There is a "2011 Live" section there

owiknowi is 100% right there

I guess this is with Pardus after you have use the USB-install version to install it.
And how is the USB-Live version doing with respect to shutdown

1.1gb should fit nicely on a dvd or an, e.g. 2gb, usb drive, afaik?  
sorry, i'm probably just causing confusion here, better stick too the people who were actual helping!

No, it's the same.
The only requirement is that the computer can boot from an USB device.
And not every computer can do that.

Every >Pardus 2009.1/2009.2 and 2011 iso file< Live and Install can be used to make a bootable  USB device.

actually i meant the most recent pardus 2011.2 which also comes as a live iso (you can use them on either a dvd or an usb)

just another thought: did you try the live dvds (32 & 64 bit) to see what happens? do they boot properly ?
is all -or most- hardware detected? and last but not least: can you reboot or shut down from such a live session?