My dad got the 9700, and after playing with it (well... working... he's new to the CrackBerry, so I ended up showing him this and that) and well figured, to jump on the beta boat again. Currently rolling 5.0.0.411 and so far its rolling great, except, for the randomly turn off and on again. Now I've SCOWERED the phone for anything that would do it, but nothing.
Did you upgrade from 4.6 or did you wipe and start over? .348 was famous for random reboots but .411 is 99% solid except for an SMS bug and a minor flaw with the icons in the camera app.
Wipe and try again. Don't forget to do the AT&T browser fix.
It just happens... though I just found out why... it seems my Auto Off times were... wrong, the off times were all at 11 in the morning O.o change and so far, no more reboots.
I upgraded from 4.6 and this is actually the ONLY problem I have with 5.0