PC Keeps shutting down

My PC keeps shutting down when it is running intense operations such as file conversion virus scanning even smart edge selection in PaintShop Pro. Task manager shows 100% CPU activity. It simply stops and I cant restart without totally disconnecting and reconnecting the power supply.
I have checked CPU temperatures on restarting after a failure and the highest I have found has been 62. Normal opeation is 55. Motherboard highest is 42 normal about 38.
I have cleaned all dust and fans out and remounted CPU and fan with new heat paste and remounted memory and other connectors.
I have used memtest86 and this failed during test6 in the same way. - presumably due to the cpu working hard. The memory has passed other tests.
I have used a different power supply with the same fault appearing.
Asus A7V8X-X KT400/Athlon 2400XP/512Mb PC2700/Hercules 3D Prophet 4500 64Mb/SB Audigy ZX/160Gb Maxtor ATA/Windows XP Home
Thanks for your help but Im stumped what to do next.
Richard

 

hi,i looked up ya motherboard and its got

ASUS C.O.P. (CPU Overheating Protection)
ASUS C.O.P. is a hardware protection circuit that automatically shuts down the system power before temperatures go high enough to permanently damage your CPU.

is the setting set too low so its turnning off even though its not in any danager.I know this is unlikely solution but if ya havent tried it, it might be it

 

No Its set to 85

 

maybe your harddrive is overheating or faulty?
each task youve described involves alot of disk activity
does it crash when u use scandisk or defragmentor?