FWIW: I found a 2820 used to reboot if wan2 was enabled but had nothing plugged in / switched on. Not sure if this feature is across the range in the newer devices or not.

Varies really.

Anything from your Syslog as suggeseted?

I've replaced a BT Business Hub with a 2860 a week ago and it's been rock solid. Sure the VDSL2 link has been lost a couple of times. Before they were rebooting the BT Business Hub every couple of days. I upgraded to the latest firmware as a matter of course.

Normally the 2860's are rock solid uptime wise, any issues are normally Exchange related.

One thing to try as I have seen on a couple of installs the line type trying to change between ADSL and VDSL if set to Auto, this causes a reboot.
This is for no apparent reason apart from something on the line as the exact same device has been moved onto a different line and works fine.
The answer to this is to lock it to either VDSL or ADSL dependant on your line type.

The only time this causes issues if you are moving from ADSL to VDSL, in which case the line type needs to be returned to Auto.

I normally lock the line type once installed.

I would like to give a hint also. Make sure that you assign unique port setup for various services in System Maintenance >> Management is correct. I had port 8443 for HTTPS and didn't notice it was also in CVM.
After making the changes didn't notice any reboots.