I'm happy to have found this thread. I was desperate.
Bought the device a couple of weeks ago and having this kind of issue since the beginning.
I updated to the new firmware the first time I ran the device, now I'm seeing it was a mistake.
I tried all possible changes in the configuration of the device, like turning off the USB HD, the network, activating/deactivating the multimedia library and so on, trying to sort out the root of the problem, with no success.
The system reboots just after a couple of songs or a few minutes of slideshow.
The unit even reboots after 10 photos or so of the Picasa internet service slideshow.
It doesn't reboot while an internet radio is playing though.
My media library is quite large too, and I often select a high level folder to randomly play some music/photo
So a possible connection of this issue with the shuffle playback mode looks like something that deserves to be explored, because I'm using the random feature all time, so I'll try some experiments this evening.