It is NOT OVERHEATING!
I have had the covers off and cleaned the filters on the fans from time to time. Nothing inside the case runs hot to the touch. Yesterday the machine ran perfectly and glitch free from before 9am until I went out at 7pm. Today, it has ground to a halt just once, within a couple of minutes of power up. I was in Ubuntu and running Firefox and playing Utube video. I reset it and it has been fine since. I am using it to do this reply.
I was advised when I built the machine to put in a good power supply. I bought a new 550 W Corsair unit. I have measured the voltages on all of the rails, they all stabilise quickly and stay rock steady. Just for amusement sake I measured the power going into the PSU and found it to be only 90 - 105 Watts , a good portion of that is driving the cooling fans. I did try a different power supply, for a week, a couple of years ago, a 450 W Corsair unit from another machine. No different.
Later I will take out one of the memory sticks and see what happens. The mother board doesn't have any built in graphics capability so I need to keep the graphics card in. Currently there are no other cards in place. I did have a wireless network card inserted but I have changed it out for a USB wireless link, it has not made any difference. I can boot up from either a USB stick or a DVD so I can run with the hard disks removed and the tray carrier unplugged. I don't see what else I can remove and still be able to run the machine up.
Rans6......