Today, just before 4:00p CDT, both of my TiVo Premieres froze and rebooted. They were both recording only one channel (Olympics on our local OTA NBC affiliate). On one of the Tivo's, we were just watching live TV on another channel. Without any warning, the picture froze, then a minute later, the TiVo rebooted.
I wanted to make sure that the other TiVo hadn't been interrupted and found that it had also rebooted about 5 minutes earlier than the one we were watching.
I didn't observe any sort of power issues at the time. They just rebooted with no warning.