My 950 (AT&T version) reboots quite often and that hasnt changed no matter which preview version was installed. I'm hoping the upcoming firmware update will help.
In my case, it has only rebooted (as far as I can tell) when the phone is bumped. This happens most frequently when I set the phone down. It sometimes happens if I jostle the phone a bit when picking it up. It has also done it when it is in my pocket and bumped something (like the car door when getting in/out of the car for example).
I've actually managed to go a couple days before without it rebooting if I make a conscious effort to be very gentle with it. This also happens whether or not I have wifi enabled so I dont think it is the wifi related reboot.