I've had two spontaneous restarts with this beta - one just last night.
I tried to file Feedback (it is still "compressing", along with the two issues I tried to file on iOS 10.0, so, I guess they were never sent..). But "Dark Sky", which I installed last week, showed up a lot in the crash logs, so I just uninstalled it. Let's see if that helps.
To update: In the 9 days since I uninstalled Dark Sky, I've had no spontaneous restarts.