Rebooting, nothing helps

I've run the gauntlet and yet...



S3 HD with known good HD, known good power supply, good fan, plugged directly into power, traded out multistream cable card just to eliminate that and no joy, still rebooting. Unit has OS 11m...



What else causes spontaneous reboots?



Lifetime Tivo so I don't want to hit it with a hammer and yet I do.



Thanks for any ideas.



JP



BTW: No indication prior to reboot, no pixelization, no pattern that I can tell.