pc keeps clicking off?

Hi There. I have an emachines 5220 which developed a fault.It would just click off.It started perhaps once a day,but has got worse+worse,now its every time I turn it on!! The monitor is fine and when it clicks off the power suply is still running and sometimes the yellow light on the front stays on.I have stripped it down and took the motherboard out to look for any loose or burnt bits,even took the processor out but all looks fine.I re-assembled it with a new power supply and I even bought a new hard drive.While downloading the xp program from the discs all went well and I thought I had fixed it,but as soon as the xp program was complete and got to the desktop,off it went!!!
It even happens in safe mode, but by pressing "del" to get to cmos/bios it stays on all day!! until I exit into the xp prog. then off it goes again!!! I may be wrong but ime 100% sure its a motherboard fault. Anyone got any ideas????
Any help greatly appreciated.