Why does my iMac keep restarting?

Hi everyone,

I have a problem with my iMac randomly rebooting.

I have already replaced the harddisk with one equipped with an OWC thermal sensor, I replaced the power supply because it could be a problem deriving from the power supply. Finally I replaced the video card with a new 2GB HD6970M.

Currently the restarts are random and I have noticed that connecting an external display the iMac does not restart.

I have also performed the Apple test in depth several times but the result does not report any errors.

What could be due to this annoying problem ???

I thank you in advance!