Had a similar problem when I upgraded my 8120 to the first time where it would randomly lockup or reboot. Tried re-sending service books and a battery pull, but it didn't help. Finally I just decided to re-download the OS and reinstall it. After that, no problems with YouTube or the reboots. Of the millions of lines of code written during an upgrade, it's totally possible one line here or there got "misplaced".