What else could be the problem.

I have three identical series 3 (TCD648250B) lifetime Tivos. Recently, one of them started in a reboot loop. It never gets past the "welcome, powering up" screen. The other two series 3s still work great.

I wound up swapping the power supply, hard drive and fan from a working TiVo and into this non-working TiVo, but none of it solved the issue and the unit STILL reboots.

Any other suggestions on what might be going on here and how to fix it?