I know this may sound silly. It may be a several things, the mother board not
reading the memory, CPU, or Mother Board. Some times a walking wounded I.C.
can cause much head ache. My hunch is there is a walking wounded I.C. and with out a test program and test board. Trying to ferret out the problem can be problem manic.

Appreciate your help but would XP have work fine when it was dual
booted if those were not working, especially the IC ?

An answer to that is yes, and maybe. There is a possibility that Windows
doesn't load down
the memory like Ubuntu does, thus causing the system to crash.

But 10.04 is said to run on 256mb while this computer runs 1gb RAM and I
do use graphics.

Have you considered trying 10.10 or 11.04, or even another distro all
together?

Yes, yes and yes on all both Ubuntu versions but not the another distro,
loath to give up on Ubuntu.

I had a similar problem with a windoz machine. Memory tested good but replaced
it with RAM from "Crutial" and problem solved.

Glad it worked for you, this RAM is new from Crucial.

gpu support and good functionality is always kinda iffy. very possible for
a buggy driver in linux to be causng problems when things are going ok in
windows. another problem specific to toshiba and a few other models is CPU
fan control. In that by default the fan does not come on, potentially
causing a reboot if the trip sensor does not start it when temperature hits
critical. Just a few thoughts.

Ah fan control is worth a check again, drivers are another thing
entirely, often i have found Ubuntu does not run graphic drivers well
and changes have to be made.

I understand that the fan control problem relates to laptops rather
than desktops - probably due to power saving circuitry that Toshiba
use.

Interesting that the crashing is so intermittent too - have you
noticed it relating to particular programs run on the days it crashes
opposed to days it doesn't ? Still trying to get a handle on what's
going on here - bearing in mind that WinXP work fine on it. Did we
ever discuss graphics chipset ? Reason for asking is that older Intel
graphics is a known issue with all flavours of Linux, not just Ubuntu.

I was having problems with my desktop rebooting on its own and
discovered that the cooling fins under the fan were clogged with dust
bunnies. The interesting thing is that this computer is dual booted and
it would only do this with Ubuntu, Windows was fine.