PC in using half-way and restart again

PC in using half-way and restart.

Sometime is able run it till 2-3 hours long and restart suddenly.

but sometime just 10 to 15 mins will restart.

Especially when CD-ROM is running, it will within 2-3mins get restart.

1stly suspect is power supply problem but not. After change a new one problem still remain.

Unplugged CD-ROM, problem still same.

After format system problem still remain.

Lastly, after change CPU then everything in fine.