Thanks everyone.
I'd not really considered the power supply as the issue was so completely predictable.
It's a Phenom 550, 4x1Gb DDR2, two sata drives, IDE DVD drive, and the graphics cards are ATI HD4670 and Nvidia 9400GT. Power supply is a good quality Acer 450 However, I'll give DaiNasty's suggestion a go and see what happens.
Any suggestions of what mobo problem could cause this?
Difficulty is I without being able to narrow it down do I buy a new mobo and a new PSU, or some new memory or a new graphics card? Might as well buy a new Dell