That is a very unusual situation. Have you ever
installed a version of
Windows, just to see if it does the same thing?

I use Ubuntu, not Kubuntu, (but they are not that
different except in the
default shell), and never had that problem. I really
question whether it is
an operating system issue... I suppose you have
checked your settings for
Sleeping and Hibernation?

The only time I ever had a similar problem was many
years ago when I bought
a computer which had a defect either in the memory or
the mainboard, causing
it to freeze several times an evening. The builder (a
small company) finally
fixed it but never told me what the problem was.

I use kubuntu and a low powered older computer without a problem.

Its not that causing the shut down but drivers can cause trouble.

I don't have a Windows disc to check with but I took the machine to a local
computer shop for them to check for me as I ran out of ideas and they tested
everything apart from the HDD's. They used Windows (with one of their own
drives). Needless to say I was not happy with them for doing such a
half-hearted job as I told them they could wipe the smaller of the two drives if
they wished (that was when I was using an 80GB and 160GB drive rather than the
250GB and 750GB drive I now use). I have checked power settings but don't think
it is those as it has sometimes happened during installation. I have run the
sensors software but it doesn't seem to be a problem with temperature as it
never seems to get very hot.

I think your CPU is going down hill or your power supply is bad.

I have replaced the CPU and PSU so both are brand new. I have even changed the
mains lead and tried a new mains socket.

One idea is installing Ubuntu, and later turn it to Kubuntu,
using SYnaotic. I have this at home and run ok.

Have you scanned for virusii? I know they are rare in Linux land, but they do
exist. I say that because one of the first virusii I had to deal with would not
let my Win98 system shut down. I know that is the opposite of your problem, but
it is still unexplained power behavior.

If your computer is turning off then I would look at:
* your plug in outlet
* your electric cord
* the air flow into your computer
* how hot your computer is running

They way you are describing it - it doesn't sound software related to me. With
all the components that you have replaced surely a hot spot would have been
spotted though. Of course changing out all those parts, maybe something is not
seated properly.

There is no virus at all with KDE. When you test any distro at any desktop
version that is going to happen. Firefox is or can be tied into the kernel of
any version of linux. Both KDE and Gnome can be very unstable. Heck when Dapper
came out. Firefox was so unstable it crash all the time. It was mainly due to
the flash player plug in's. If you DL Firefox flash blocker. You'll see crashing
was the was the pass. All so put in mind that Linux is open source. I cannot
recall paying money for it.
Linux is growing and grow with it. I been using it for 5 years and been getting
better from each up grade. When it is time to up grade to the new version. I
wait for couple of weeks and read up on the bugs.

It sounds like a hardware issue as opposed to an OS one.

It could be a problem with the power supply. I would open up the computer and
blow it with a can of compressed air and see if that resolves it. If you haven't
done this lately this could be the source of the problem. Or if you have added
hardware and your power supply is not powerful enough it will act like this,
too. These things can be erratic so it is hard to see a pattern and attribute it
to one specific thing.